Help Android O update "No command"

[wade poh 97]

Hi, I'm using Nexus 6P. Early this morning i received the OTA update for Andriod O beta program. But after the installation and reboot, the phone showed up "no command". It probably needs to factory reset, but i want back my data.
So I decided to apply Android N update via ADB to recovery my phone, but now i couldn't install the ADB driver for my phone. It required "USB debugging" enable. But now i even cannot boot into OS how do I enable the debugging?
Any way to install "ADB driver" without usb debugging enable??
Please advice. Thanks

 

[doniago]

I'm hardly an expert, but it sounds to me like you may be borked. If your O install changed any files them I'm not sure you can go back to N (the beta program info says that while you can go back to N your data will be erased in the process).