Most likely, after a couple of days of using the test macOS Big Sur 10.16, you will want to return to the stable macOS Catalina 10.15. On the 2020 MacBook Pro, I managed to do this using almost the same system menu.
So, you installed a brand new macOS 11 (how to install macOS Big Sur 11), but something went wrong. For example, you are left without important apps and can not replace them with anything, or you have realized that life without iTunes is not pleasant to you. You won’t be able to return to macOS Catalina 10.15 in a couple of clicks, but it is quite possible if you are willing to spend a few hours deleting the system and restoring it.
How To Downgrade From macOS Big Sur 11 To macOS Catalina 10.15:Since we will reset the Mac to factory settings in the process, you will lose all your files if you do not make a backup of Time Machine. Alternatively, you can send files to Dropbox or OneDrive if your computer is new and you haven’t grown a lot of everything important yet. It is good if you made a backup before updating. If not, do it now.
Step 1: Start the system restart.
Step 2: During it, press and hold ⌘Command + ⌥Option + R. This will take you to the transition menu for the latest macOS compatible with this computer.
Step 3: Now go to the “Disk utility”
Step 4: Use the “Erase” button and mark the APFS format + “Erase” again.
Step 5: After that, select “Reinstall macOS”
Step 6: Follow the instructions of the installer. You will also lose all data on the disk.
So, you rolled back to the old version of the system, now you just need to return your data. Restart your Mac again and hold ⌘ and R. Select the “Restore from Time Machine backup” option. Confirm your choice, specify which disk contains the data to restore, and select the latest available Mojave backup.
If you uploaded the necessary files to the cloud, just download them back and install the necessary software.
Done! You successfully downgraded from macOS Big Sur 11 to macOS Catalina 10.15.