Pavajeje you can do as follow:
1. As Paola says: Remove battery (maybe you already done it
)
2. download official Pocket Book 15.8 firmware from
here
3. Unzip the package and copy SWUPDATE.BIN file in the root of the SD card
4. Insert the SD in the reader and switch it on
5. During the boot process press and hold OK and FORWARD buttons simultaneously until you will see a "Firmware update" message on the display
At this point firmware update procedure should take place so follow instructions.
Hoping this helps!