First, create a Time Machine backup or back up all your files to iCloud. Then, to erase macOS Monterey, restart your Mac in Recovery Mode, choose Disk Utility, and select Continue. Highlight your hard drive and click Erase. Select your file system (usually AFPS) and click Done when the process completes. Next, to install Big Sur from Recovery Mode, restart your Mac in Recovery Mode and follow the prompts. Alternatively, install Big Sur from a bootable USB drive.