There are two adjustment screws along the outside edge of the lens. One will be along the top center and the other along the inside center. One of the adjusters is for left and right adjustment and the other is for up and down.
Also, in 2003, GMC and Chevrolet use the slide pins to secure the headlamp assembly. When they get installed sometimes, they don't latch at the bottom as they should. As a result, it's nearly impossible for adjusting the headlamps. See pic 1 below. Make sure the assembly is tight and properly installed. Also, when you Install the headlamp assembly retainers, push down and inward to secure the lower portion of the latch.
Let me know if this helps or if you have other questions.
See pic below.
Saturday, July 10th, 2021 AT 9:33 PM