You need to loosen the three 19mm bolts on the back of the compressor. you can see them easily from under the car.
The from the top, there is a long stud with a 10 mm nut on it. You will find this on top of the compressor,just under the steering pump. Loosen this nut, it is the "adjuster" for the compressor. After you install the new belt, remember to retighten the three big bolts on the back of the compressor.
Oh, you will also need to remove the steering belt to be able to get the a/c belt off.