I have a Nexus 9 tablet I bought a few years ago. One day a software update caused the tablet to become unusable. It was stuck in a loop showing the Google logo and restarting. I few weeks ago I was able to revive it after searching various posts on the Web.
- Install ADB. I have a Mac, so I used Homebrew to install this tool:
brew cask install android-platform-tools - Hold down Volume Down + Power Button. This gets you into HBOOT. It can be tricky to accomplish.
- Press Volume Down to highlight Recovery, then press the Power Button to enter it.
- Wait until the Google logo goes away, and you should see a picture of an Android laying on it’s back. Hold down the Power Button + Volume Up (very important to hold Power FIRST).
- On the Recovery page it lists a few details about your version at the top. Note down the 6 character identifier for your device (which is in the format of 3 text characters, 2 numeric, and another text char). My identifier was n4f26q.
- Go to https://developers.google.com/android/ota and find the OTA image that contains the identifier. Download the image.
- On your device use Volume down to get to “Wipe Cache Partition”, and press the Power Button to activate it. This takes a long time.
- Plug your tablet into your Mac.
- On your device, use Volume up button to go to “Apply update from ADB”, and press the Power Button to enter it.
- On tour Mac, type
adb sideload <file-downloaded-in-step-5>. - Wait. Mine took about 40 minutes.
The bulk of the above instructions came from https://forum.xda-developers.com/nexus-9/help/failed-marshmallow-update-stock-android-t3226704.