uninstall mac Catalina

There is no uninstall or downgrade.

if you need to reinstall a previous OS you will need to do so by wiping the drive with Catalina first or on a blank partition. Doing this will DESTROY all data on the drive so make sure your'e backed up first.

if you don't already have a time machine backup you can reinstall the OS that shipped with your computer

https://support.apple.com/en-us/HT201314

to migrate the data from your previous OS

https://support.apple.com/en-us/HT204350

You can't "uninstall" it.

What you can do is restore your system from a time machine backup made before you upgraded to Catalina.

Why do you want to do this? If you're having a problem, we might be able to help if we know what's wrong.