My car has a sagging headliner, how can I fix this? It's pretty bad.

 

You can repair it by removing the headliner board from the car, stretching the fabric and using 3M adhesive to hold the material back on tighter. There are multiple posts and such about this -- use the search technique. Or, you could buy a whole new precovered set if it's real bad. Or, you could also just buy new material and recover your existing shell.

 

kerryismygirl is correct.

the easy way is to buy a replacement headliner and throw it in.

if you'd like to save $$, remove the headliner from the backing, spray the backing with 3M adhesive, and carefully stretch the cloth over the backing, making sure to leave no wrinkles or bubbles in the fabric.