need to download Big Sur SOFTWARE

You would need a Late 2013 or newer MacBook Pro as noted from Apple menu () > About This Mac. Then visit How to download macOS - Apple Support using the Safari browser. This will deposit the full Big Sur installer in your Applications folder.

Would add - have a Rescue Plan and in-place plus often used Before upgrading - just incase things go sideways.

3 Backups using 2 Different Methods and 1 Off Site should a Natural or Un-Natural Disaster occurs

Make Time Machine Backup  is very useful  and is a good start