OK! I am under the impression from looking on DC Rainmaker that I can exchange the basemap in a garmin 520 with a more detailed one from ‘Open Street Map’. So I signed up and received the download of the map I wanted in a zipped file. My question is do I un-zipp it before transfering it to the Garmin or will the software in the Garmin recognize it and do everything for me? Has anyone done this? how did it work for you? from the standard basemap would you recommend the change? Many thanks

you need the file within

The easiest way: get hands on a precompiled (=ready to use map), e.g. by following a link in the wiki. This map will be called GMAPSUPP.IMG

Set your unit into mass storage mode (connect it with a computer via USB), or take out the card and copy the file with a cardreader onto it (see 2).
copy the GMAPSUPP.IMG file to the folder GARMIN (or create this folder if not available) with your operating system (you do not need Mapsource or any other dedicated software for this step).
unmount the USB-Device from your computer (eject in the filemanager)
https://help.openstreetmap.org/questions/4592/basic-guide-to-download-osm-to-garmin-gps