Here are the steps to update your Nexus 6P to Android 7 (final) release.
Step 1. Install Android Studio
Download and install Android Studio from here. If you think you already have everything you need, at least verify your adb version like so:
.. Check adb version
Android Debug Bridge version 1.0.36
Step2. Download the full Android 7 image for the Nexus 6P
mv ~/47ceb0f81a49e6866428f9996f8212aba8c29fbe.zip nougat.zip
Prepare your phone
Make sure Nexus 6P is in developer mode, has USB debugging and (just in case) OEM unlocking enabled.
Boot the nexus 6P into recovery
adb reboot recovery
When in the infamous ‘No command’ screen appears, press-and-hold the Power button then press-and-release the Volume-Up button, to get to the standard Recovery menu.
Navigate to ‘Apply update from adb’ and select with the Power button.
Push and flash the zip from your computer
(if no connected device is shown unplug the USB-C able and plug it back in)
adb sideload ~/nougat.zip
Eventually, the standard Recovery menu will reappear. Navigate to ‘Reboot system now’ and select with the Power button again.
All done .. Android 7 now runs on the Nexus 6P