The firmware upgrade procedure for the jetBook is documented on this page of the Ectaco web site:
http://www.jetbook.net/upgrade.php
I have found that a few minor changes to their procedure works better for me. To begin with, before starting the procedure, reset the jetBook by sticking a paperclip into the little hole on the back. With the jetBook off, insert the SD card with the firmware files.
At the beginning of "Step 1" and "Step 2" you have to hold one or more keys while powering on the device. I find that you should release the power button (while still holding the other buttons mentioned) as soon as the display turns on. If I continued to hold the power button, the device would eventually turn off, which is not what you want.
I will admit that the firmware upgrade procedure is rather complicated and could stand some simplification. However, the above two tips should make it a bit easier.