the quick and dirty just to get going would be to find a posting for an unlocked topo image of the area you're interested in. change the filename to gmapsupp.img if necessary, copy that file to a directory named "garmin" on a microSD card, insert it to the gps and turn it on. done.

Although this is the easiest path to updating your maps, it is also the least flexible method because you're stuck with (sort of) whatever specific data somebody else decided to include in that map file.