I presume it is like the old days: dowload the firmware, unzip it on an SD card (or on the device if you have not got an SD card), connect the device to your computer, then switch it off, then back on. While it is switching on, press both of the buttons with the arrows, until "updating firmware" appears on the screeen, at which point you release the buttons and watch the machine updates itself for the next couple of minutes.
Here are the instructions:
http://download.pocketbook-int.com/6...ook-902-en.pdf
after you have preformed the update, it is better to delete the three files that you unzipped on your device, as otherwise in future updates the device will first check the updating software available on it, and go for that one unless you tell it otherwise.
EDIT: OS= operating system
Flashing: you overwrite the existing software of the device with a later (or also the same) version. There is only one part of the memory of the device that you can see when you connenct your pocketbook. A lot of the software is hidden, and to access it you have to "root" the device, which is loosely speaking like getting administrator privileges on your computer.
This is the computer illiterate explanation of these terms