1. Use a tool to remove the two screws from the lamp assembly. 2. Remove the lamp assembly. 3. Press the release tab and turn the bulb socket counterclockwise to remove it from the taillamp housing. 4. Pull the old bulb straight out from the socket. 5. Press a new bulb into the socket, insert the socket into the taillamp housing and turn the socket clockwise into the taillamp housing until it clicks. 6. Reinstall the rear lamp assembly.
It makes more sense with the pictures in the owners manual. If you don't have the manual and would like one, you can download one FREE at: http://www.autousermanual.com/view/gmc/