un-downloading catalina
how do i un-download catalina back to mojave? photoshop CS6 won't open so need to revert back to my old software
MacBook Pro 13", macOS 10.15
If you made a Time Machine backup prior to updating see here: Revert your Mac to a previous macOS version - Apple Support
Otherwise, you will need to wipe the Mac and install macOS from scratch, which is quite a bit harder to do.