A car roof lining that sags is a real detraction from your car's appearance. This is a simple fix, however, if you have the proper knowledge. Replacing a head liner in the car's interior is not a job for an upholsterer or another specialist. Even the newest do-it-yourselfer will find the task manageable. There are many types of liners and each requires a slightly different approach.
First remove all the parts and accessories from the roof. This includes the coat hangers or overhead handles for passengers. Unscrew the hardware attaching them to the roof with a Phillips head screwdriver and remove them from the roof entirely. Some cars have a rear-view mirror attached to the ceiling, if so, this must be removed as well with a screw driver. Pry off all the connecting tabs with a flat head screw driver, and remove the trim and moulding with a small pry bar. The existing lining should pull right off of the roof, after you unsnap the plastic brackets and tacks that secure it in place. Pull the edges free of the roof and the adhesive cement that holds them in place.
After the old liner is removed the new one is set in place. First, make sure the liner is the right size by measuring it with a yard stick or ruler. Dry fit the piece in the car if you feel this is necessary. Finally, the liner area is prepped for installation. Cement or upholstery glue is applied to the seams on the edges of the roof with a glue gun. The back of the liner is installed first. Stretch the fabric forward covering the entire section of roof as you install. If there are plastic clips to hold the upholstery, install the fabric into them. Then snap the tacks into the plastic brackets to secure the headliner fabric around the surface area of the roof.