this isn't a very good picture of the bolt itself, but it will help you locate yours. It sits on the end of the leif spring at all four corners of the car. It has a rubber pad on the bottom end of the bolt that rests inside the A arm. The bolt needs to be backed off counter clockwise, raising the pad up closer to the spring. this allows the A arm to come up closer to the spring. If you take them all the way up until the pad stops, you'll lower about 1". I've also heard of (vett) people triming off the bottom of the pad, making it thinner, but 1" is really enough. GM calles these "trim adjustments" and their intention is to level the car. The bolt actually has about 1" play on top and bottom of the spring, so you could even raise you car up to an inch if you want that jacked up rear end look, but I hope not!
Helpfull hint: After you jack up the car to get the wheels off the ground, there is still pressure on the pad from the A arm. Spray lots of oil on the pad to allow it to turn easier.
Recomend a 4 wheel alignment when you're done. I never did on my '04 and never had issues, but it's been suggested to me. I will on my V, but my dealer couldn't do it....it was too low to get it on their racks!