It's probably a bit late, but if you keep the power button pressed for 5-10 seconds the device will force power off.
Here are instructions to reset it
https://help.kobo.com/hc/en-us/artic...-Kobo-Aura-H2O
Note that an update can take a little while.
But for any future readers, I meant just stick to your working install.
Uncontrolled should normally just overwrite everything, that is, in effect disable KSM.