How can I revert back to pre-mac OS Catalina?

If you have a Time Machine backup, use Time Machine to revert back to an earlier release. Otherwise, you cannot.

However if you would like some help on why you want to revert back then please state what the issue(s) is.

As rkaufmann87 said, if you have a pre-upgrade backup "all" you need to do is boot into the Recovery Partition, delete the two container drives Catalina created, format the hard drive and then restore from the backup. If you don't have a TM backup or clone it is a much more arduous process. But it is possible we can help fix your Catalina 'problem' so you won't want to move backwards.