There is small button on the rear panel of the media player (U button). You'll have to search it.
Before using it, you have to switch off your player.
Insert your firmware on a USB stick (install.img), put it on your player.
Disconnect your media player, press the U button, don't release it, reconnect your media player.
Switch on with the remote control, without releasing your U button.
When you'll see your firmware upgrading beginning on your screen, then release your U button.
Good luck !