GMC 2008 Yukon Denali Owner's Manual page 28

Unfolding the Seatback(s)
To return the seatback to the upright position, do the
following:
1. Open the liftgate to access the controls for the seat.
2. Pull up on the seatback until it locks into the
upright position.
CAUTION:
{
If the 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 seatback to be sure it is
locked.
3. Push and pull on the seatback to make sure it is
locked.
1-24
Tumbling the Third Row Seat
The seat can be tumbled forward for additional cargo
space.
To tumble the seat, do the following:
1. Open the liftgate to access the controls for the seat.
2. Make sure the head rests are completely lowered,
there is nothing under, in front of, or on the seat,
and all items are removed from the cupholder
and storage bin, if the seat is a two-passenger seat.
3. Fold the seatbacks forward using the instructions
previously listed under "Folding the Seatbacks".
You will not be able to unlatch the seat from
the floor unless the seatback is folded down.