Ok, I've re-found
the page. It seems complicated, since I've not a SD reader and I'm not happy about opening the device.
I noticed that Kobo Glo has a pin hole, next to the USB port. It should a factory reset pin. If something goes bad I think I could use it and restore the software, copying the 2.4.0 firmware, downloaded from the direct link, and all the contents of the fat32 partition I previously backed up. Or not?