For testing purposes, perhaps you can set up a password protected subdomain and a create new 'slot' on your web hosting service. It should be possible under cPanel.

Make a backup of the live site. Copy the live site onto the testing slot. Upgrade the board on the testing slot, making notes of the steps.

If the upgrade is successful, take the live site offline and make a backup. Keep it as a snapshot.

Create another copy of the backup. Replace the testing site with this latest copy of the live site. Perform the upgrade as per the successful test run. When it's working correctly, replace the live site with the upgraded site. Switch on the live site. The DB and files will be in sync.