GMC 2011 Sierra Denali Owner's Manual page 56

3-2 Seats and Restraints
Head Restraints
The front seats have adjustable
head restraints in the outboard
seating positions.
WARNING
With head restraints that are not
installed and adjusted properly,
there is a greater chance that
occupants will suffer a neck/
spinal injury in a crash. Do not
drive until the head restraints for
all occupants are installed and
adjusted properly.
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.
Pull the head restraint up to raise it.
To lower the head restraint, press
the button, located on the top of
the seatback, and push the
restraint down.
Push down on the head restraint
after the button is released to make
sure that it is locked in place.
The head restraints are not
designed to be removed.
The rear seat has headrests that
can be adjusted up and down.