Slide a plywood board under the cushions to provide support. This will help if your cushions are in good shape but the springs in the base of the couch are sagging.
Unzip the cushions, and remove the inner foam pillow. Measure the pillow, and buy a new one at a fabric and sewing supply shop.
Slide the new pillow form into the cushion cover. It may take some manipulating and pulling on the fabric to get the pillow inside. It will be easier if you work one corner, then the other, pulling the fabric toward the back.
Shake the cushion back and forth to settle the foam cushion inside, then zip shut. Replace on your sofa.
Rotate your couch cushions every few months to prevent excessive wear on any one spot.