GMC 2015 Sierra Denali Owner's Manual page 309

10-38 Vehicle Care
2. Remove the two rear lamp
assembly screws.
3. Pull the rear lamp assembly
outboard away from the box side
until the retainers release. There
will be a noise when the
retainers release.
4. Pull the rear lamp assembly
straight back to remove it from
the vehicle.
5. Turn the bulb socket
counterclockwise.
6. Pull the bulb straight out from
the socket.
7. Replace the bulb, then insert the
bulb socket into the rear lamp
assembly and turn clockwise.
8. Verify the retainer ring is in the
proper position. If the retainer
ring is out of position, it will not
engage. Reset the retainer by
pulling it forward with a tool.