how do i downgrade my catalina os to mojave

Restore the backups created prior to the upgrade.

Revert your Mac to a previous macOS version - Apple Support

Restoring a clone also works, and some folks will create those clones prior to any substantial upgrade.

The only other path is erasing and re-installing the older macOS release. Any files that have been updated as part of macOS or as part of updates or upgrades associated with versions of the apps for the newer release may or will not be accessible from the earlier versions and the earlier versions of the apps.

macOS and most other software upgrades are one-way, and there are typically no downgrades.