my macbook has been o heating up since downloading Big Sur

If you would open the link supplied earlier, it would provide all the Reliable Instructions requested.

To spare you time - a copy and paste verbatim from that link

Revert your Mac to a previous macOS version

If you use Time Machine to back up your Mac, you can easily revert to a previous version of macOS if you experience trouble after installing an update. To select a Time Machine backup from an SMB server, your Mac must have macOS Sierra or later installed.

Important: Reverting erases any changes you made to the files on your Mac since you installed the new version of macOS. To save new or revised files, copy them onto a different disk or back them up before you follow these instructions.

1. Print these instructions.
2. Click the Share button  in the Help window, then choose Print.
3. On your Mac, choose Apple menu  > Restart. After your Mac restarts (some Mac computers play a startup sound), press and hold the Command and R keys until the Apple logo appears, then release the keys.
4. Select Restore from a Time Machine Backup, then click Continue.
5. Select your Time Machine backup disk.
6. Select the Time Machine backup you want to restore. To restore your computer to the state it was in before you installed the new version of macOS, choose the most recent backup that occurred before you installed the new version.
7. Follow the onscreen instructions.

If you backed up files as described in the Important message above, you can restore them now. If you backed them up using Time Machine, click the Time Machine icon  in the menu bar, choose Enter Time Machine, then retrieve your files.

If you reinstall a previous version of macOS on a Mac or partition, new backups will use that version of macOS. If your backups are stored on a Time Capsule or other network storage device, existing backups are retained; they aren’t overwritten by the new backups.

Hey leeuwin,

Thanks for being a part of Apple Communities.

Having your MacBook exhibit more heat than you're expecting can be alarming but if you're willing, there's some tips we'd like to try and go over for this kind of concern to see if we can't help with that without the need of more drastic options like reinstalls.

Find out what to do when your Mac is getting warmer than expected in Keep your Mac notebook within safe operating temperatures.

Run through that article and try the recommendations presented and feel free to return back here should you have further questions.

Regards

Revert your Mac to a previous macOS version