The
only map your Garmin can read is this one:
[SD card]\Garmin\gmapsupp.img
so you can obtain a new version of gmapsupp.img from wherever (such as Andy's page) and then one easy way to do it is with a card reader, if you have an adapter for microSD cards. Just replace the existing gmapsupp.img (maybe back it up first) with the new one, using your usual file manager.
If you don't have a card reader you can connect the GPS and coax it into 'external drive' mode via
Menu -> Menu -> Setup -> Interface -> USB Mass Storage
and use file manager as before - but this is a lot slower if it's a very big map file.
If Andy's files are too big you can obtain smaller OSM maps for custom-selected parts of the UK (and the rest of the world) from here:
OSM Routable maps