GMC Envoy Owner's Manual page 13

Pull the seatback toward you as you lift up on the lever.
The head restraint will automatically fold out of the
way when the seatback is folded down.
To raise the seatbacks, lift up the seatbacks and push
on them until they lock into the upright position.
Push and pull on the seatbacks to make sure that they
are latched securely. Then fold the bottom seat
cushion back into place.
1-6
The rear seatback levers
are located on the
outboard side of the
rear seatbacks.
To return the head restraint to the upright position,
reach behind the seat and pull the restraint up until it
locks into place. Push and pull on the head restraint to
make sure that it is latched securely.