Power device on. Wait for a moment between booting of ros starts till it is not started yet. Unplug power. Plug power again and the second partition starts if it was set so… Using standard reset procedure you can get default admin login.
I guess a reboot is necessary for repartitioning the storage. The copy from live to backup partition is without reboot. But you should then boot to the second partition to test it. At least. So two reboots need to be expected. Try it in your lab first on non productive device to get familiar with it.