The easiest method is to erase the drive and reinstall macOS Big Sur from your Time Machine Backup, secondarily with another backup.
If you don't have a backup and want to restore your data, that is much more difficult as you must erase the drive to install a previous OS.
If you don't have a backup, but you are running Catalina or Big Sur, that won't be too bad as you can keep that volume and try to migrate from it. However, the Library formats are likely different, so you would have to export from your Libraries prior to importing them back into the old OS.
If you have no backups and came from Mojave or earlier, you'll need to extport all of your data to an external drive before erasing and installing Big Sur. Same rules apply regarding new Library to old Library--You'll have to export all of the data, then import it into the new library, losing all keywords, playlist, etc.