The tutorial to upgrade is: omv-release-upgrade
To upgrade "without impact" is very difficult from OMV 2.x to 3.x. Impossible to write a tutorial for everything that could go wrong. I recommend a fresh install. But since you have a backup, you can always try it. If it goes badly, restore from backup.
Ahaha, I just did a backup 5min ago so right now if something goes wrong I just need 10min to restore my set up with clonezilla.
That being said, I would like to do that when you are available just in case! Is right now a good moment?
So just need to execute in CLI the comand: “omv-release-upgrade” ? I always though it would come on GUI as something friendly for the user