Replace the fabric
Take the headliner out and pull the old material off carefully. If you are careful, you can sand the headliner smooth, just don't stay in place very long. If there are any cracks, fix them first. I used hi-temp duct tape that has super adhesive backing. Go to a auto paint store and buy the 3m spray adhesive (contact cement) in a spay can. You will probably need 2-3 cans. Spay both the headliner material and backing, allow to tack, then start attaching the parts together. You will need a helper for that part since you will need to get the material straight. As you attach the headliner smooth it out in one direction, only do one half at a time, that seems to work better. Allow to dry for a few hours, trim, and reinstall. It is not that hard, I've doing my own headliners for years. The hardest one I did was on my hearse, I saved $1000.00 by doing it myself, and most people think I had it done at a upholstery shop. Just take your time, you will probably do just fine.