Maps are archived with 7-zip http://www.7-zip.org/
Unpack the map in a convenient place on HDD. For Windows Vista or newer to install click at batch "install_MAP.bat", where "MAP" is the name of downloaded region. Batch will create symbolic links from Garmin directory to the map. Installation process doesn't copy any files, program on PC will use your unpacked files.
Install batch automatically unistalls previous version of the map without removing map files.
For older Windows you can install map manually. In this case you need to copy whole *.gmap directory to a valid Garmin maps directory. To find valid directories run "maps_windows.bat".
Maps are prepared by Garmin Mapconverter. Here you can find instructions for installing maps under Mac OS X:
You will need Garmin MapManager, which can be downloaded here:
After installing MapManager find location where you unpacked the map. Double click on *.gmap directory to install.
Installation of the map doesn't remove any previous version, this should be done manually.