There isn't a button behind the card flap on mine
1) Power off the receiver
2) Ensure no other USB devices are plugged into the receiver
3) Insert the USB stick containing the extracted image file
4) Power the receiver on.
5) When the two colour power button flashes the display will show the active partition.
6) Use up/down on the remote to select which partition to flash and then touch the power button to start the flashing process.
7) If successful the receiver will reboot itself after the flashing process has completed.
8) Remove USB stick that contains the images files.