It's a pain in the ***.
Pull (hard & suddenly) at the black trim hiding the rear of the brake light until 2 or 3 of the clips pop off allowing you access to the back of the light, through the metal frame of the car. Be sure to heat the trim up if it's a cold day. Then get a torch and look in the 2 big holes in the body, find 2 long slits in the light's grey plastic frame and use 2 flat head screwdrivers to move those slits along to release the light. It then pops out from the outside, left (nearside) first I believe as the wiring harness is on the right. Have a magnetic grabber or long pliers handy as 4 metal clips will fall off and need to be picked up, as well as the light's rear frame which yo just removed with the flat head screwdrivers.
To refit a new light (or the original), put the backing of the light on in the unlock position, use a little tape to hold it there, then slot it back into the car, line it up so it fits snug, then slide those slits back into the lock position. Refit the trim (you might need to take off the positioning clips and straighten them up) and you're done. Took me 15min to replace my brake light last month, but I had a friend to help too, it made a difference. (I did try about 9 months ago, took the entire boot trim off and then realised Audi had sold me the wrong boot light!)
