[fredjacksonsan]

On the idler arm (the wheel with nothing attached to it) there is a slot on the back side that you can get either a 1/4" or 1/2" (forget the size) socket DRIVER into. It's pretty hard to find. Once you have it in the square hole, you can turn it (look first to get direction) to release the tension on the belt. Helps to have someone take the belt off while you're holding it.

Before taking it off check to see if there's a routing diagram for the belt under the hood, if not draw yourself one to be sure.

After new H2O pump is on, installation of the belt is reverse, put the belt almost all the way on using diagram, use the driver again, and put tension on the idler arm. Pull the belt into place, making sure that the belt is on all the pulleys correctly (all grooves matching)

This is likely a good time to replace the belt also, so if it show signs of wear (cracks in grooves or missing portions of grooves) get a new one.

Hope this helps out.