To repair the sagging headliner in a car, use professional spray adhesive, foam-backed fabric, a soft wire brush and a single-edged razor blade. Begin by removing items mounted to the ceiling, such as map or dome lights, sun visors and clothing hooks, and any trim holding the headliner up.
Remove the headliner, being careful not to bend or crease it. Remove any old foam from the headliner board using the wire brush. Position the new foam-backed fabric on the board, coat the foam backing with spray adhesive, wait until it is dry to the touch, and then lay the fabric onto the board. Use the razor blade to trim the edges. Rub the fabric to help it stick to the board, and replace the headliner and any components removed to access it.