Introduction
The serpentine drive belt should be replaced every 65,000-105,000 miles.
Follow this video tutorial by YouTuber NutzAboutBolts to replace the belt yourself.
Conclusion
To reassemble your device, follow these instructions in reverse order.
CONTAINS NO INSTRUCTIONS! Directs you to YouTube to watch his videos. Should be removed.
Mike McAvoy -
This is a video embedded guide and can be watched here on iFixit or on YouTube.
Blake Klein -
Just did this replacement last night. Bought the belt from O'Reilly and rented the serpentine belt toolset. It didn't come with a 14 mm socket, but I was able to borrow one they had. Fairly easy to remove the belt, but putting it on, especially in the dark, was nontrivial. I recommend doing in the daylight.
Be sure that the tensioner springs pushes back while you're starting; it may need to be replaced too.
I had to really reach my arm all the way down to get the new belt on, and then be creative to get it around all the right pieces. Perhaps a longer socket would have helped for being able to push the tensioner back and still have space to work with the belt.
Water Ski -