What will happen if I flash the Pixel Android P Device image on my Nexus 6P?
This is the easiest way to flash Android P to your Nexus 6p. Although it is possible to keep all your files, I recommend a backup in case something goes wrong. 


Install Nexus Root Toolkit on your PC. This will allow you to flash anything you want.
Next, download an image of Android P for the Nexus 6p onto your desktop. All images for the 6p are available for download there. 
Now connect your 6p via USB.
Click on "Flash Stock + Unroot" on the toolkit. Select the image you just downloaded (not the images the toolkit offers since those are outdated) then choose whether you want to factory reset the device or not.


Now the toolkit will automatically flash the image you chose onto your machine.