I take it you were using the PC Cable is that right? It is recommended for units with the card slot to update via SD card.
Put the firmware on the SD card and put it in the card slot on the 798. Turn the unit on and it should automatically detect the new firmware version. Follow the instructions, making sure not to cut the unit off until the update is done.
If, it does not recognise it, you may need to send your unit in for repair.
Robert