replaceing alternater how do I release the tension on belt 3.9 L V-6

Try this: Rotate the tensioner counterclockwise. Push the spring clip toward the tensioner to lock the accessory drive belt tensioner in place. Picture: http://bit.ly/drive-belt-tensioner You might find a free repair manual here: http://bit.ly/autozone_repair_info

Pull hard on serpentine belt with two hands to clear alternator pulley or remove the three alternator mounting bolts one at a time to remove belt. When reinstalling alternator install the single upper bolt to use as a pivot, rotate the alternator into place with the belt on the pulley, then install the two lower parallel mounted bolts last. Hope this helps, and these methods do not require any use of the hard to access tensioner.