When you first buy a car or truck, the foam cushions that make up the seats should be firm and hold their shape. Over time, however, the foam cushions may sag and begin to disintegrate. If your cushion is over 10 years old, starting to have a rounded front edge, or no longer offers you support, it's probably time to replace it. For many cars and trucks, you can order a replacement and do the job yourself, however, depending on the model of your vehicle, you may need to take it into an auto bodyworks garage or an upholstery shop to get the job done.
Measure the dimensions of your foam seat. Take the height, length, and width. Note the shape and size.
Order a replacement foam cushion. You will need to provide the dimensions of your existing seat, as well as the shape, intended use, and weight of the person most often using it.
Examine your seat cushion. Many seats have Velcro, snaps, or buttons that hold the seat cover over the foam insert. If the fasteners are difficult to unfasten, use a slotted screwdriver to pry them open. You may also need to use needle-nosed pliers to get the fasteners loose. Consult the service guide for your vehicle for more information about your seats.
Remove your existing foam cushion, and insert the new piece of foam. Make sure it fits snugly in the seat cover.
Replace all the fasteners, and tighten them for security.
If the seat cushion fasteners are too difficult to remove, or you can't easily find them, contact a mechanic or upholsterer to help you. You may need to remove the seat entirely, which is a job for a professional auto specialist.