for the F900BT, you need to be able to press the reset, eject, menu and map buttons all at the same time to initiate.
Looking at yours, I'm guessing the procedure is most likely to be different.
quite possibly then, as i have no eject button
I believe the update process is different for the 500, I wouldn't want to recommend something I haven't or can't try out myself, if you want a link to the map files you are more than welcome, but not too sure how you would get them on the unit, sorry