If the rear seat in your 2002 Pontiac Grand Prix folds down, it'll be a split-bench seat. In the trunk behind each rear seat at the bottom of the seatback, you'll find a pull tab. Pull it and push the seatback forward.
Don't Unless You Have To
Pontiac doesn't recommend driving with the rear seats folded down unless you have a need that requires you to. The danger is that anything in the trunk can slide forward during an emergency stop or a collision. Once the objects get loose in the passenger compartment, they can injure you or your passengers. You should lubricate the folding seat hardware at last once a year, using an aerosol lubricant -- GM part No. 12346293 or equivalent.