Deleting the Mac-Os Data Partiton

The Macintosh HD - Data partition contains all of your files and data, removing or erasing this will erase your personal content.

Since you are coming from the Big Sur beta back to a public release of macOS, you will need to completely erase the hard drive, including the Macintosh HD - Data partition. Obviously it is best to backup your data with Time Machine before carrying out any such process.

Click the Macintosh HD - Data partition in the left side bar, click Edit in the menu bar and then click Delete APFS Volume. Then follow the steps at How to erase a disk for Mac – Apple Support for erasing.

For information on Catalina's system partitions, see About the read-only system volume in macOS Catalina - Apple Support.

For information on reinstalling macOS from a bootable installer see How to create a bootable installer for macOS – Apple Support

Best of luck!

If you have a Time Machine backup you can revert to Catalina without losing all your files etc.

https://beta.apple.com/sp/betaprogram/restore#macos

ShadowDust99 wrote:

You should not need to remove the Data partition to reinstall macOS from a bootable installer, it should simply reinstall macOS onto Macintosh HD and leave your data unaffected.

FWIW, that's actually incorrect. You have to erase the drive first to downgrade.

Apologies, I noticed after I posted that the original poster was coming from Big Sur beta. When composing the response I thought they were simply trying to reinstall macOS from a bootable installer. I edited the post just before you quoted by the looks of it (nonetheless my post is still incorrect for advising the OP to backup from Time Machine while using Big Sur).

Apologies GourishRC, follow the steps which dialabrain posted, not mine! Let us know how it goes.

Best of luck!