How to uninstall macOS Big Sur beta

Revert your Mac to a previous macOS version - Apple Support

Use Command "R" at start up and using the Mac OS Recovery Utility restore using your Time Machine backup, or by reinstalling Mac OS Catalina (another choice on the utility). You may actually have to use the dis utility option first and erase your Apple SSD drive first...then go back and restore from back up or by using the link to Mac OS Catalina.

I have a 2018 Mac mini and updated to Big Sur Beta. It behaved badly and I tried to go back to Catalina from a time machine backup. I wound up in an endless loop of downloading OS from internet and then going back to the options screen of download from internet, disk utility, etc. I erase the Mini's SSD and tried again. Same result. I tried to get into the T2 security which ask's for a firmware password. When I click on it I am told (without entering anything) that the firmware password was not found. Any ideas?

I was in that endless loop on a 2019 MacPro. At first he had me do a PRAM reset (Option - Command - P - R ... all 4 keys at the same time, wait for the second chime). That did not help me. Apple support had me erase the Apple hard drive. I was not really erasing the one I needed to. Then we rebooted into Recovery (Command R...until you see the spinning globe). Then I was able to select my time machine backup and backup from there).

If you run into problem, feel free to post back.