I have got one of these boxes but I use openatv.
You can only flash from the rear socket.
When flashing openatv you insert the flash drive power on wait for the display to change from boot to USB and press the standby button.
The display then shows flash and the box flashes.
I think it is different for openxiv try this.
Insert drive, press and hold standby button while powering on.
I am not sure whet happens next but I think the display says flash or maybe USB and you release the button.