Typically you want to take a backup of a root filesystem that is not live. That means either booting from other media or moving the media for the root filesystem to some other computer. And then copy the not-live root filesystem. Either using rsync or using disk imaging.
But often a full backup is not needed. All that is needed may be to backup the configuration and be able to restore it. Often that means just /etc.
There are desktop tools to backup live root filesystems, and restore them, that greatly reduce the need for a full system backup. Timeshift is one example. I run it on my Ubuntu laptop. You run Timeshift before an upgrade to make a new snapshot of the root filesystem. If something goes wrong you can use Timeshift to revert back and reboot to the state of the system from before the upgrade. Timeshift is mainly based on rsync, but also uses some "tricks" to more fully be able to restore things like /boot and mbr.
I would welcome the addition of a similar functionality to armbian-config. To be able to update system snapshots and to restore them at will.