The belt is tensioned by a spring-loaded tensioner pulley. If you can fit your arm in there far enough to pull on the belt, you should be able to see that pulley move. That's the one you need to move to release the belt tension.
Many of these have a 1/2" square hole on the arm that will accept the drive end of a 1/2" breaker bar or ratchet. Others have a 15 millimeter bolt that holds the pulley to the arm. What worked best for me was to put the box end of a combination wrench on the bolt head, then hook another box wrench onto one tooth of the open end of the first wrench. This gives you more leverage so you don't have to tug so hard on the wrench.
Another tip, if you are not replacing the belt, is to keep tension on it by pulling it up from the power steering pump pulley when moving the tensioner pulley, then move it off the pump pulley and release the tensioner. Now, use a bungie strap to lightly pull the belt up and hook the strap to a hole in the bottom of the hood. This will keep the belt centered and in the proper routing on all the other pulleys.
If your new pump does not have the pulley installed, you will need to transfer the old one to the new pump. This requires a pulley puller and installer kit. Before removing the pulley, note how far the hub sticks out from the end of the shaft. This distance is critical. Installing the pulley mispositioned by 1/16" can result in an incurable belt squeal. New pumps and new pulleys from the dealership might come with a spacer that must be used with the pulley installer tool. There are three possibilities; a small spacer, a smaller spacer, or no spacer required if the hub of the pulley was flush with the end of the shaft. If you don't have the right spacer, a less desirable way to do this is to look at the inside of the pulley hub for rust. The rust will provide a " witness mark" to show how far to install the pulley.
Do not use a hydraulic press to push the pulley onto the pump shaft. Pressure on the shaft will break one of the rotors inside the pump. Some new aftermarket pumps come with a long bolt and nut to use as a pulley installer. Often, these bolts are not hardened and the threads can pull out easily. To avoid this, be sure to thread the bolt as far as possible into the pump shaft. If you need a spacer and one wasn't supplied, visit your friendly dealer. Many techs save the spacers that come with replacement pumps and pulleys rather than throwing them away. The spacers will usually work with the installer bolts supplied with aftermarket pumps.