Uninstalling the OS

You would need to erase your Mac's Hard Drive entirely and use a USB installer to install macOS 11 Big Sur.

click here ➜ Erase your Mac and reset it to factory settings - Apple Support

then here to download Big Sur ➜ How to download and install macOS - Apple Support

Do note, there's been an issue recently with the activation server for older versions of macOS, and it may not validate the installer and may not let it proceed. If that happens you will need to wait and try again later and see if its working.

No, erasing the drive removes the installed version along with all files and data on the computer. It will reboot into nothing. You will need to boot the machine into recovery mode or choose the previously prepared install USB from the boot screen options if it appears.

Make sure to backup any files and data you want to keep.

click here ➜ Create a bootable installer for macOS - Apple Support