I believe the XM is a rebadged Huricane A2E/ RadioOcean 4500
in which case the procedure is:
Turn radio off.
Hold down  and [watch] while turning radio on.
Enter 4500 as a pin (think thats done by rotating the channel selector and pressing it in to accept)
You should then get an option to reset MMSI. Push channel button to accept then confirm...
Switch radio back off and on.
If you need a german dealer: Busse Yachtshop: VHF DSC Radio Ocean 4500
(English manual on there tells you how to enter a new MMSI.).