GMC Sierra 2015 Owner's Manual page 67

3-6 Seats and Restraints
Do not have a seatback reclined if
the vehicle is moving.
Manual Reclining Seatbacks
Warning
If either seatback is not locked, it
could move forward in a sudden
stop or crash. That could cause
injury to the person sitting there.
Always push and pull on the
seatbacks to be sure they are
locked.
To adjust a manual seatback:
1. Lift the lever.
The seatback will automatically
fold forward.
2. To recline, move the seatback
rearward to the desired position,
then release the lever to lock the
seatback in place.
3. Push and pull on the seatback to
make sure it is locked.
To return the seatback to the upright
position:
1. Lift the lever fully without
applying pressure to the
seatback, and the seatback will
return to the upright position.
2. Push and pull on the seatback to
make sure it is locked.
Power Reclining Seatbacks
To recline a power seatback,
if equipped:
Tilt the top of the control
rearward to recline.
Tilt the top of the control forward
to raise.