GMC 2010 T-Series Owner's Manual page 26

5. To make the lap part tight, pull up on the
shoulder belt.
It may be necessary to pull stitching on the safety
belt through the latch plate to fully tighten the lap
belt on smaller occupants.
1-22
To unlatch the belt, push the button on the buckle.
The belt should return to its stowed position.
Before a door is closed, be sure the belt is out of the
way. If a door is slammed against a safety belt, damage
can occur to both the belt and the vehicle.