How to Change serpentine belt on 1995 Pontiac sunfire?

This is a very common question. I had to take mine off and put it back on when I replaced my alternator.
One of the pulley looking things that the belt goes around, is a tensioner. It seems to be located in a couple of different places, dependent on which engine you have and perhaps which build of either engine. Mine (a '96 SE) has the tensioner located just behind and down a bit from the alternator. You can recognize it by it's shiny polished surface. At the center of the tensioner pulley is a nut. I think mine is about 13mm. You can put a wrench or a socket on it and rotate the entire tensioner away from the belt, thus loosening the belt. While holding the tensioner, pull the belt off the other pulleys. Installation is essentially the reverse.
However! This is really a job for two guys. It's nearly impossible to hold the tensioner with one hand and locate the belt with the other. Also, make note of the routing of the belt. My car has a diagram under the hood. Yours might or might not have that diagram. If not, Make a drawing of the belt routing! It's imperative that you get the routing right. (This is one case where my Haynes manual didn't really help me much, but I suggest you buy one anyway.)
When you have the belt back on, start the car and watch the belt for a few minutes just to be sure that everything is aligned properly and that the belt doesn't jump off.

Good luck