I don't have a DIY, but I recently opened mine up. I have both a basic (non-dimming, no homelink, no compass) and a version with all the features. They were slightly different but both came apart the same. All along the perimeter, about 1/4" back from the mirror itself, you will see a groove. This is where it separates. There are snap clips all around which hold it together. You need to put a flat head screwdriver in there and twist it to pop it apart. It doesn't go easy, but I didn't break any clips. I started in the middle on top or bottom, it didn't matter. I actually used a trim removal tool (it's plastic) and did almost no damage to the mirror as a result of the prying. Good luck!
2009 335i M-Sport: BMW Performance SSK/pedals/ spoiler/suspension/rear diffuser, 650i clutch, 35% tint, JB4, AFE DCI, Meth Injection, Ceramic coated VRSF Downpipes, ER Chargepipe, DTEC Angle Eyes, LED Interior and Reverse Lighting, Shift Light & Low Meth Light inside Rear View Mirror Clown Nose