How do you remove bmw expansion rivets?

If it is a rivet that is put on with a standard pull type rivet machine, (I have heard rivets reffered to in many terms, just not expansion but that is what rivets do to stay in place so forgive me if I mis-undertood), that is just like the standard rivets you can install with a hand puller then it is easy.

You have to find a drill bit small enough to make sure you don't damage the surfaces riveted together. Give yourself some space just in case you go off center or don't keep the drill straight. Once you drill through even if the is some meat left on the rivet, you can use a pair of snips to cut off the bottom if accessible and the top usually comes right off. If you can't access the bottom of the rivet or the top does not come off, a chisel or a small screwdriver will pull up an edge of the top and pop it off.

If you need to you can also take the drill bit and move it around in the hole till it comes loose. You don't have to remove all of the material to get the rivet out as the head of the rivet is all you need to weaken enough to remove and usually getting it within a few millimeters of materials left it will come off without much trouble.