Best way to remove the buttons on most Mercedes pushbutton assemblies (the ones made by MK) is to remove the bottom cover and press them off from the rear. Make sure you give even pressure as you can break the switch shaft if you are not careful. I usually use two small screwdrivers or a pair of thin long nose pliers.
Your unit is the Bosch however. By uncliping the front cover (4 places) you can apply pressure to the cover to pop the buttons off.
Replacement buttons are available from the dealer parts department, however there are lots of scrap units around. Know any Mercedes mechanics? They usually have a few under their workbench.