I swapped out my paddles in my 335i to M3 paddles. Not a bad do it yourself.
Step 1 Disconnect battery so the airbag doesn't go off
Step 2 feel under the side of your steering column for little divets. Insert a jewelry flat head screwdriver, press against the spring and push the airbag up. Do the other side. This step is a pain in the ass.
Step 3 tape the airbag up so you don't have to disconnect it
Step 4 unscrew the steering wheel cover
Step 5 unscrew the paddles and disconnect
Step 6 reassemble the paddles with the new paddles
Step 7 put it all back together
This was from memory, should be pretty close.