Ok, you have a link. I'm not saying that this is going to work correctly the first time.
Find your current slot or just use both.
Flash rec.img to recovery.
Code:
adb reboot recovery
-- should boot to a white screen after Onyx/Boox logo
adb shell
-- got ADB? With a # ?
id
-- does it say "root"
start autoupdate
-- did you get a nice "Waiting for flash"? Legible? Right side up?
-- put update.zip (decrypted) on a flash drive or an SD card and stick in device
-- does it run? To the end? Happily? Did it reboot to your new system?