How do you loosen tension on drive belt fo the alternator?

There is an automatic tensioner that keeps tension on the belt. THere should be a label in the engine compartment somewhere giving you a diagram of the path of the belt and showing where the tensioner is and which way it turns to loosen the tension. On the 1995 GMC Sierra that I just worked on, I loosened the nut on the front of the tensioner a little, then I used a big pipe wrench on the tensioner itself to rotate the tensioner assembly counterclockwise then removed the belt while still holding the tensioner, then gradually released the tensioner. From looking at it I guess there is a special tool you can use to do this, you might want to check with Advance Auto. or whoever your closest auto parts store is to see if they will loan you the tool.