MattH,
It depends on the board your player has.
Please be wary when using other people's fimware on your player as even though the casing looks the same, the chip inside it might be completely different! Flashing your player with an incompatible firmware will render your player '
dead' in most cases.
If you have an ACTIONS board (a player that plays .AMV video files), then you can read the
Knowledge Base - "
How to: extract firmware of your player using the S1fwx tool" from
here.
If you have another board (Sigmatel/Sunplus), I haven't got instructios for that yet. I will post it soon.