First off, it's a belt not a chain unless it's a "vanos unit". If you are mechanically inclined and have sufficent tools - it's an involved process & I'd advise getting the Bentley Service manual for the step by step procedures. If you're not sure of your abilities or tools required I'd have it done by a dealer or qualified shop. Hopefully it's not already gone bad - if so you're going to need more than a belt. (The valve tolerances are very tight and if your belt has broken, it's 98% likely it's also done some valve damage) Sorry to be the messenger