The entry-level Audio20 infotainment system has the optional Garmin Map Pilot navigation on SD-Card. The SD-Card contains both the navigation software and the maps data. Normally, you'd buy a new SD-Card that will have both the latest version of the navigation software and the latest version of the maps. You can 'hack' the SD-Card and copy onto it data downloaded from the Internet, but this 'hacking' is obviously not officially supported by either Mercedes Benz or by Garmin. As said, the normal process is to simply buy a new SD-Card that contains both the latest software and the latest maps.
The higher-end COMAND infotainment system, on the other hand, does not make use of the SD-Card slot for navigation, instead the navigation software and the maps are both stored on an internal hard drive. With COMAND, you can update the maps separate to the navigation software, and vise versa.