Change the serpentine belt on a 2000 Buick Regal

The tensioner should adjust and take the correct belt and tension it correctly. The reasons for belts coming off and worn / loose belt, wrong belt, tensioner not working, or incorrect belt alignment.

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You'd better hope you have a belt routing picture under the hood. Usually the belt is released at the tensioner pulley, which is spring loaded to keep the belt tight. The pulley is unloaded by using a 3/8 long handled socket wrench. Find the square hole.

Serpentine belt replacement is not complicated and requires only one tool. Find the tension pulley located at the bottom of the motor. This pulley is attached to a very strong spring and holds the belt tight against the pulleys through its serpentine route around the engine. Use a socket wrench with a matching socket the same size as the hex head bolt in the center of the tension pulley. Attach the socket and set the direction to tighten the same direction the spring action allows the pulley to move. Turn the wrench handle, the pulley moves and the tension on the belt slackens. Take the belt off the pulley and let it hang. 
To install the new belt Once the belt is around the pulleys allow it to hang freely at the bottom of the engine. Again, use the socket wrench to turn the tension pulley far enough so the belt can be wrapped around the pulley. Once the belt is around the tension pulley, release the socket wrench and allow the tension pulley to take up the slack. 
Good Luck!!

if you're not mechanically inclined then don't attempt it , but if you are then get a 3/8 long handled ratchet or breaker bar and locate your belt tensioner simply push the ratchet or breaker bar into the square opening in the tensioner and pull toward front of car or push towards back of car and you will fell it give you shouldn't have to push hard once this is done make sure you have a firm grip and pull belt off top pulley which ever is closer and easier to access then follow diagram on front above radiator or on strut tower

ALL serpentine belts have a tension pulley which holds the belt tight to the various equipment it turns (water pump, alternator, a/c compressor, etc). Find the one spot where the belt goes over a component that has nothing connected to it, and you have found the belt tensioner. The tensioner is designer with a nut on the face of it so that you can put a socket wrench on it; by 'unscrewing' the socket wrench, you should be able to release the tension on the belt - it will take some pressure to release. Once you have tested this, you can pull onthe socket wrench and remove the belt; replace it witha new one and take off the socket wrench. Make SURE you have the correct belt and be careful with the socket wrench; it takes lots of pressure to keep the belt tight.

You have to turn the tensioner pulley clockwise using a ratchet wrench with a 2 or 3 foot bar attached to it for leverage to get the belt off. Then install the new belt according to the diagram sticker at the front of the engine compartment.

use a ratchet and 15mm socket and make sure the belt is routed properly mak sure the all the grooves are in line

This is the procedure for 3.3 and 3.8L engines:

1. Disconnect the negative battery cable.
2. Raise the front of the vehicle and safely support it with jackstands.
3. Remove the right front splash shield.
4. Release tension by rotating the tensioner clockwise.
5. Remove the belt and install a replacement.
6. Proper belt tension is maintain by the dynamic tension.
7. Install the right front splash shield. Lower the front of the vehicle.
8. Connect the negative battery cable.

Firt - study & write down (draw) the belt path,,
and be sure about it..

Then - put a wrench on the nut in the center of the tension pully.. Move the tension pully to loosen the belt.. Pull belt off..

Repeat to put new one on..

Should be pretty easy..

One pulley is a tensioner. Put a wrench on the nut in the center of the pulley and relieve the tension from the belt.