There is no downgrade available.
You will need to restore your pre-upgrade backup to revert, or to erase the Mac and clean-install Big Sur.
If there are no pre-upgrade backups and this is an erase-and-reinstall, you’ll then have to manually transfer the documents back from the Monterey environment that have not been altered by the Monterey install and apps. Documents that have been modified may or may not be readable on older versions.
Installing beta software without backups and in production doesn’t always end well.