The serpentine belt is held tight by a spring-loaded tensioner. First, take a good look at the belt routing. If your memory is like mine, you might even want to draw a picture. Grab the belt on a long straight spot. Pull on it to see which pulley moves. That is the tensioner. Using a long wrench for leverage, move the tensioner until you can slip the belt off one pulley, then relax it. Pull the belt off, Put the new one on, and your done.