GMC 2009 Envoy Denali Owner's Manual page 14

Head Restraints
Adjust the head restraint so that the top of the restraint
is at the same height as the top of the occupant's
head. This position reduces the chance of a neck injury
in a crash.
1-10
To lower the head restraint, press the button, located
on the top of the seatback, and push the head
restraint down.
Pull the head restraint up
to raise it.