Here's a couple threads from different websites with some details:
http://www.dieselplace.com/forum/sho...63&postcount=8
http://www.gm-trucks.com/forums/inde...ded&pid=893750
It's relatively simple. There is a 5.5mm bolt that holds the lid of the cabin filters to the hvac box. Take it out, take the lid off, and commence pulling the filters out. Be sure to have a trash bag or a drop cloth on the floor directly beneath the hvac box. It'll get messy if it hasn't been changed it years. There will be two, they are side-by-side.