Restore previous-released OS version(mojave) from catalina public beta
Unlike some installations of some operating systems there is no OS level uninstall feature in macOS.


That is true for applications, though some do have uninstallers available. 
That is true for minor OS version upgrades, EG 10.14.1 to 10.14.2 
That is especially true for full versions, EG 10.14 to 10.15


What that means is that if you install an OS update or upgrade and you need to revert back to the previous version there is no built-in way to do so. 

In fact the ONLY way to revert  any OS upgrade is to back up your files, boot into recovery mode (or a bootable USB installer), wipe the drive and re-install the operating system. Once that is done you can restore from your backup.