Go to google's support page for the device and download the stock image and you can flash that via ADBGot a Nexus 6 for Christmas, got it activated and started the root process today, but I messed up big time due to going down the "traditional" root path without realizing that the update released this morning no longer supports it. Oops...
I made a post asking for help in the appropriate XDA thread, but would anyone here happen to be a wiz at rooting?
https://developers.google.com/androi...s/images?hl=en
Stick with chainfire's systemless root option as it patches ramdisk on the go and will provide better compatibility with all the different Roms out there. If you download older roms you need to check to see if they using a prebuilt supersu package as they require a special sepolicy to run otherwise you will bootloop. There are unsu.zip files to remove the older version and flash the newer one if necessary.
Chainfire Releases Systemless Root For Android 6.0