Pull the sagging interior fabric away from the edges of the car roof. Use a utility knife to make a clean cut along one side to free it from the foam. Peel it away to the point of the sag.
Spray a layer of upholstery adhesive to the foam that was beneath the fabric. Hold the can approximately six inches away, and move it in a back and forth motion until the area is covered.
Reapply the roof fabric by gradually pushing it into place over the glue, pulling tight as you go along. Smooth your hand across the fabric as you go along to eliminate any air bubbles or wrinkles.