If you enter the class and year of your car into this link you will get a page where you can download any firmware updates.
Home @ Mercedes-Benz Connectivity
I think all you need to do is save the download to your PC, then burn a copy to a CD and let it run in your Audio20.
If you don't want to do this yourself, a little further up the M3 in Lightwater, Comand Online can assist. The phone # is in the link below.
Catalog