upraged to Big Sur how can i downgrade to Mac Catalina or Mojave?

Howdy,

First you will want to backup your Mac to save all your files.

Then follow the instructions from How to reinstall macOS

which includes:

• On an Intel-based Mac that previously used macOS Sierra 10.12.4 or later, you can use Shift-Option-Command-R at startup to install the macOS that came with your Mac, or the closest version still available. 

You may also want to refer to Restore shipping software

which assumes you have a Time Machine backup done before a beta install, but worth the read.

ivan

Also check out, How to get old versions of macOS