If it's the same as the Audi A6. Easy job bud. My tensioner was broken, so I couldn't adjust it.
Under try off, you can then get to the compressor. I had to undo the bolts and just release it from the belt that way. You can put it back on this way too. Worked fine. It will then lower down just enough to work on it, use an axle stand to support it. If it's the same, it's a 10mm bolt in the middle. Then you need to pry off the metal cup over the inner pulley. Once that's off, you need circlip pliers to remove the circlip holding the inner pulley. Once that's off, pull off the pulley. Push on the new one, circlip on, outer pulley may need some lube to go back over the rubber. Put some good loctite on the 10mm screw (mine actually came out and the pulley disapeered completely!).
Put the compressor back on, refit undertray. Took me about half an hour.