This article explains different ways to downgrade from macOS Catalina (10.15) to Mojave (10.14).

Time Machine is macOS’s built-in backup utility. And if you want to roll back to Mojave after upgrading, it can help you do that too. If you find yourself downgrading macOS shortly after upgrading, you can restore a backup from before the change. Unfortunately, this technique has a time limit; you have to use it before the program dumps the old backup.

Connect your backup drive to your Mac.

Restart your Mac while holding down Command+R.