If possible format the USB when you put it in the computer. If you do not know how to format, Google is your friend!!!
Download the firmware file and copy it to your USB
Remove the USB from your computer and insert it into your STB
Important-Back up any user data onto your usb using the "dump by USB option" eg: settings , misc files etc
How do I update the firmware?
Make sure your USB is plugged in then navigate to; > Menu > Tools > Upgrade by USB > Keep Upgrade mode as "img file" > Select the firmware you have downloaded > If you have a keep dbs settings select "no" (you will need to update channel list after this update" > Click on "Upgrade" > If a warning appears just click "Yes" > After it has updated the box will reboot > A channel list update is recommended. Please see "channel lists"