GMC 2009 Yukon XL Owner's Manual page 9

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 head rests that can be adjusted
up and down.
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