Google’s latest Android update for the Nexus 6 is not going to plan. After the latest security update was found to cause issues with Android Pay, and verifying the system as a CTS certified device, it looks like Google has bungled it, attempting to downgrade users from Android 7.1.1 to Android 7.0 and causing all sorts of issues in the process.
The /r/Nexus6 thread on reddit is showing multiple users have received the OTA update which forces the phone backwards to Android 7.0. The process of actually downgrading an OS update can be time-consuming and resets are required, which doesn’t appear to be happening – causing apps to crash and generally making life unpleasant for Nexus 6 owners who accept the update.
The update(downdate?) is apparently intentional with a Nexus Community Manager chiming in to advise :
There was a recent OTA update for Android 7.0 that is causing confusion for some Nexus 6 users. For those of you that want to guarantee future OTA updates, you will need to get back on the 7.0 supported track. If you are experiencing issues after accepting the 7.0 update, please factory reset your device to help resolve any problems.
For any Nexus 6 users that want to continue flashing/sideloading future builds, you can stay on 7.1.1 and continue to do so.
Hope this clarifies! Nexus Community Manager
If you’re a Nexus 6 owner and are happy to continue flashing/side-loading your updates, then ignore the update prompt, but, if you want to continue getting the updates then a downgrade and factory reset are in your future.