It's not always such a good idea to download a huge update on Day One. How to downgrade from macOS Big Sur back to Catalina.
You can always downgrade back to your previous operating system. If you've downloaded macOS Big Sur on your main hard drive, you're not out of luck.
The first, and most important thing you should do before upgrading to macOS Big Sur (or updating any software, no matter how small), is to back up your Mac.