This is a quick explanation.
The Porsche 987.2 has a different engine than the Porsche 987.1 Boxster, and is slightly different to change the serpentine belt.
How different? Not very, but I just wanted to point out real quick, where the wrench goes to release the tension.
Follow all the steps to gain access to the belt.
1. Put the convertible top in the service position. Basically that means to open it half way and stop.
2. Remove the key from the ignition to stop the boing boing boing noise, and also to keep the top from moving on you.
3. Slide both seats all the way forward and tilt them forward.
4. If you have the Bose sound system, you have to move that out of the way, just like removing the storage/package tray out of the way.
5. Lift the top carpet to reveal 4 plastic nuts holding the carpeting to the rear firewall behind the seats. Undo those nuts and put the carpet upwards and put it aside.
6. There's an aluminum plate that covers the access to the belt. It requires a 10mm wrench to undo the nuts and bolts.
Step 1: The Magic 30mm Bolt
And here's where you loosen the tension on the 987.2 engine.
The casting has a 30mm bolt! Drop a wrench on there and the belt loosens quite easily!
From there it's just fishing the new belt in, under and over. It takes a little convincing to get the new belt over the tensioner, but it will fit. Just go slow, and don't try to force it with a tool.
Make sure everything is in it's grooves, and run the engine before buttoning things up. Save the old belt, toss it in the trunk, just in case.
Sorry about the finger in the shot... I was multi-tasking.