Barry,
There's great information (with step by step instructions with pictures) in the Technical Manual here. Click into it from the homepage, then Heating, then Replacing Heating Levers.
The answer to your question is - you remove the chrome faces by removing nuts which hold them to the dash. Then you can remove the heating lever assembly. The parts can be had thru any of the vendors (Adsit, etc.). I think there's even a set on eBay right now. Definitely replace all four once you have it out.
I'm not the most mechanical guy in the world, but since I started hanging out here, I got the confidence to do it, and completed mine in about 6 hours (disassembly and assembly). The instructions here were invaluable. Good luck.
Bill
71 280SL