Android Nougat is now available to Google devices and will be coming to other phones imminently. Here's how to download Android Nougat 7.1.2 beta, the latest revision to the Android OS.
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On 22 August, with little fanfare, Google released the final version of Android Nougat. It's out now on compatible Google phones and will be coming soon to other brands - also see: When will my phone get Android N?
It has in December released Android Nougat 7.1.1 for the Nexus 5X, Nexus 6P and Pixel C, with support coming for other Google phones in January.
The most recent update - Android Nougat 7.1.2 - should be available in the next few months, but a beta version is available to download to Pixel devices now.
Android Nougat 7.1.2 is said to be "an incremental maintenance release focused on refinements, so it includes a number of bugfixes and optimizations, along with a small number of enhancements for carriers and users".
The Android Nougat 7.1.2 Beta is for developers, but as before anyone will be able to install it on a compatible device. Read on for instructions on how to subscribe to Google's beta programme and install Android N 7.1.2 Beta.

How to get Android N 7.1.2 Beta: How to download Android Nougat 7.1.2 Beta

Downloading Android N is simple with Google's Android Beta Program. Any devices enrolled to the programme will receive OTA updates to the latest Android operating systems where available.
• To enroll in the Android Beta Program, open the browser on your compatible Nexus or Pixel device and head to g.co/androidbeta. You will be asked to sign into your Google account.
• Scroll down the page and you'll find a heading 'Eligible devices' with any compatible Nexus devices that are also signed into your account listed below.
Android N - enroll
• Find the Nexus device you wish to enroll to the beta programme and click the green 'Enroll device' button beside it.
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• Tick the box to agree to the terms and conditions, then tap 'Join beta'.
Android N join beta
• A message will pop up to tell you your device has been enrolled and will soon receive an OTA update to the beta version of Android. Click Ok.
Android N enrolled
• The update notification arrived on our Nexus 6 instantly, although it can take up to 24 hours. If you don't receive your update notification after that time, check you're connected to the internet then head to Settings, About, System updates and check for updates. You will not receive OTA updates if you previously manually flashed Android on to that device (as we had with our Nexus 6) - you'll get a notification that verification has failed. Instead you will need to manually install Android N on that device, and you can follow the same instructions we provide below for manually installing Android Marshmallow.
• Once you see the update notification, pull down the notification bar and choose Download.
Android N notification
• In the next window you'll be told that this will install a preview version of Android N on to your device. Ensure you are connected to Wi-Fi, then tap Download. (Note that the screenshot is of the Android N Developer Preview and not the Android N 7.1.2 Beta.)
Android N download
• You can now install the Android Nougat 7.1.2 Beta. Tap Restart & install to begin the process.
Android N restart & install
• You'll automatically receive notifications of any new Android Nougat update. 

How to downgrade Android N 7.1.2 Beta: Uninstall Android Nougat 7.1 Beta

Removing or uninstalling an Android N Beta is as easy as is installing it. You simply head to the Android Beta Program page at g.co/androidbeta then tap the Unenroll device button next to your Nexus device. Do note, however, that doing so will wipe all data on your device - be sure to back up Android first.

How to get Android Marshmallow now

Android Marshmallow is already available for recent Nexus devices and some high-end devices from key hardware makers such as HTC and LG. You can check whether an OTA update is available for your device or, if you have a Nexus phone or tablet, force the installation by flashing the necessary files using Minimal ADB. Check when will your phone get Android Marshmallow here.
Be warned that manually installing Android M is not for novice users, and it's quite possible to brick your device if you don't know what you're doing. It's important to back up any data installed on your phone or tablet before you begin since this will be lost in the process - see How to back up Android.
Below we explain how we installed Android M on our Nexus 6; follow our advice at your own risk - PC Advisor takes no responsibility for damaged devices. For those without Nexus devices wondering when they’ll be able to update to Google’s latest Android update, take a look at this: When will my phone get Android M?
Step 1. On a Windows PC install Minimal ADB and Fastboot. You can download it from this XDA-Developers thread. WonderHowTo has created a script to simplify the installation of ADB & Fastboot on a Mac, which can be downloaded from here. Once downloaded, extract the Zip and place the Android folder on your Mac desktop before opening a new Terminal window and entering the following:
Cd Desktop/Android
[Enter]
./ADB-Install-Mac.sh
The script will take a little while to run and you may have to enter your Mac account password, but once complete you’ll be able to run ADB and Fastboot commands from a terminal window. The rest of the process should be the same, but we can’t confirm as we performed the update from a Windows PC.
How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Step 2. Download the appropriate Android M installer for your device, which you'll find on the Android Developer's site. The Android M files are only compatible with the Nexus 5Nexus 6Nexus 9Nexus 7 and Nexus Player only, so do not try to install it on a different device like the Nexus 10 or Nexus 4.
Note: If you previously flashed a device to an Android M Developer Preview image, that device will not automatically get the update to the final Android 6.0 build by an over the air (OTA) update.
Android Marshmallow factory image
Step 3. You'll need to extract the contents of the downloaded Android M file to a new folder on your desktop. We used the free 7-Zip utility to achieve this. From the folder on your desktop copy the extracted files into C:\Program Files (x86)\Minimal ADB and Fastboot. (Some users have needed to rename the .tgz file extension as .tar in order to complete this step.)
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How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Step 4. On your Nexus phone or tablet open Settings, About phone/tablet and tap on Build Number seven times. This will unlock a hidden Developer Options menu within Settings. Open Developer Options and enable USB debugging and OEM Unlock.

How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Step 5. Plug your Nexus device into your Windows PC via USB and download the Google USB Driver. Extract the contents of the Zip file to a safe place, then click on Start, Devices and Printers, right-click on your phone or tablet and choose Properties. Open the Hardware tab, then choose the top entry under Device Functions and click on Properties. Update the driver, pointing Windows to the Google USB driver you've just downloaded. A prompt will appear on your device's screen to 'Allow USB debugging'; tick the box to 'Always allow from this computer', then press Ok.
How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Step 6. Now you're ready to flash Android M on to your device. If you're sure it's been backed up properly (you will lose everything otherwise), launch Minimal ADB and Fastboot. Type adb reboot-bootloader and hit Enter. This will boot your device into Fastboot mode (which can also be achieved by switching it off and then simultaneously holding down the power, volume up and volume down buttons).
Step 7. Scan the information on the device screen for LOCK STATE. If this reports that the phone or tablet is unlocked move on to step 8; if it is locked, in ADB type fastboot oem unlock and hit Enter. Use the volume button on your phone or tablet to select Yes, then use the Power button to confirm your choice.
How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Step 8. Technically, flashing Android M should now be a case of typing flash-all and hitting Enter. When you then reboot the phone you'll be greeted with Android M.
How to get Android M now: How to install Android M Developer Preview on Nexus 5, Nexus 6, Nexus 9, Nexus Player
Except this didn't work on our Nexus 6, and we received an error message that the update package was missing system.img before it aborted the process. If you get the same error messge, move on to step 9; if you don't, enjoy Android M.
Step 9. In order to make Minimal ADB and Fastboot see those files, we had to go back to the files we extracted from our Android M installer in step 3. Within those files is another Zip file, and it's in here that you'll find the missing system.img file. Extract this Zip file, then copy its contents into C:\Program Files (x86)\Minimal ADB and Fastboot.
Step 10. Rather than using the flash-all command you'll need to manually install each file. In Minimal ADB and Fastboot we entered the following commands to successfully get our Nexus 6 running Android M:
fastboot flash bootloader bootloader-shamu-moto-apq8084-71.11.img [this is for the Nexus 6 - the filename here will differ for the Nexus 5, 9 and Player]
[Enter]
fastboot flash radio radio-shamu-D4.01-9625-05.16+FSG-9625-02.94.img [again this is for the Nexus 6 - the filename here will differ for the Nexus 5, 9 and Player]
[Enter]
fastboot reboot-bootloader
[Enter]
fastboot flash recovery recovery.img
[Enter]
fastboot flash boot boot.img
[Enter]
fastboot flash system system.img
[Enter]
fastboot flash cache cache.img
[Enter]
fastboot erase userdata
[Enter]
fastboot flash userdata userdata.img
[Enter]
fastboot reboot
[Enter]
The device should then restart running Android M.

How to uninstall Android M: Revert to Android Lollipop on Nexus 5, Nexus 6, Nexus 9 and Nexus Player

To uninstall Android M and revert to your previous operating system, simply download the appropriate system image from this page and repeat the instructions above. Note that you'll first need to clear out the files from Minimal ADB and Fastboot that you added earlier.