need help Nexus 9 stuck in endless reboot loop
Bear in mind the following will wipe your device data:

To flash the firmware to your device:
While boot-looping, hold the power button until the screen goes blank. As soon as it does, hold the volume down button and your device will reboot into fastboot mode. From here, follow the instructions on the Nexus Factory Images to flash the firmware by flashing the bootloader and then the rest of the system using fastoiot on a connected PC. After this, you can run fastboot reboot if you're device doesn't automatically boot into Android.