how can I go back to the OS version before Big Sur?

Reboot holding down the four keys Shift - Option (⌥) - Command (⌘) - R

You should see this:

Select Reinstall macOS (it might refer to OSX instead) then follow the instructions to carry out the installation.

Once complete, you should have installed the original operation system for that Mac. Log on. Open Safari. Visit:

https://apps.apple.com/us/app/macos-catalina/id1466841314

and install 10.15 Catalina offered to you.

The report back here!

If you don't mind losing all your data, you can:

• Reboot holding down the four keys Shift - Option (⌥) - Command (⌘) - R
• Use the Disk Utility to erase the internal drive.
• Install macOS from the menu provided.
• .. and if the version is too old, then upgrade to the version you want.

It's more challenging to retain your files because you can't install one version of macOS over a newer one.

If you do not have a Time Machine backup of the previous operating system, you are faced with wiping your startup drive, and performing a clean installation of the previous operating system, without any means of restoring previous user data and system settings. If you don't have that crucial TM backup, I suggest you learn to live with Big Sur.

Could you tell me how to install the new macOS ? It isn't showing up as A original update on my MacBook and I'm really at A loss on what to do . Also I've been reading something about backing up the MacBook onto the Time Machine but when I clicked it I was just confused on what to do.