How to change back to Catalina?

Going back, used to be a lot easier with the older OS X versions.

It is not as straightforward using Recover in the later macOS versions.

You need to have a good backup. (preferable from before you upgraded to Catalina)

You need to startup in macOS Recovery and use Disk Utility to Erase the Internal Drive.

You need to reinstall the earlier macOS version or Restore from an earlier Time Machine backup.

check out > https://www.digitaltrends.com/computing/how-to-downgrade-from-catalina-to-mojave/

and/or > https://www.sweetwater.com/sweetcare/articles/macos-10-15-catalina-downgrade-operating-system/

If you experience any issues with Mac OS, often best way to resolve them is to perform clean install.

In this particular case this is most likely the only option.

You may check this article by Apple: How to reinstall macOS - Apple Support

There are at least two options to downgrade: you may use Time Machine backup to go back, and second one is downgrade using bootable OS installer.

Don't forget to backup!

There is also an article with more precise instructions.