Hi there !
I never had to use the backups with my previous versions of MiaB (the ones working with Ubuntu 18.04). Since I upgraded to Ubuntu 22.04, I decided (and had) to test how to restore them.
The maintenance guide is helpful (https://mailinabox.email/maintenance.html#moving-boxes), but I think it could me more precise even if I did not double-checked. From my own experience, I guess that the section “Create a new box” could be something like following :
Then follow the steps in the setup guide’s section Setting Up The Box. When you are prompted for the box’s hostname, you will need to use the hostname that you are currently using. When creating the user, choose one which had admin privileges and set up the same password.
Actually, I just tested with a new (non-existing) user : I tried neither a user without (former) admin privileges, nor a user with (former) admin privileges but with another password.