I have a 1990 ford probe, I need to change the serpentine belt, how do I lossen the belt to get it off.

To remove the belt you have to use either a wrench the correct size or a 3/8" ratchet to move the belt tensioner. The belt tensioner is one of the pulleys that your belt is riding over. It is spring loaded and holds tension on the belt. Sometimes they are hard to get to because of the tight fit of the engine to the fenderwells. If you push on your belt with your hand, you should be able to see the tensioner move up or down. As soon as you let loose of the belt the tensioner should pull the belt tight again. This is a very generic description but works on all cars and truck without having to buy a shop manual. Note that there should be a belt routing diagram on your radiator support, hood, or other article to show you how the belt is actually routed. Pay close attention to the correct routing of the belt. Some of the routing diagrams show where the tensioner is located. Hope this helps you out.