You need to replace the whole headliner. Some Stores like JoAnne Fabric's sells headliners. If you've got an odd color, they may not carry it. I've seen, black, grey, tan & dark red there when I bought mine there. It was all of $16.00 for 6 feet in legnth.
You cannot reglue you old one up. First of all the foam has dried up & turned to dust most likely. Plus even if you did shoot adhesive up in the headliner, it be so dusty in there the glue wouldn't bond.
When foam dries up & disintegrates thats when the cloth material begins to sag.
All of the trim moldings around the door openings, center pilar's, windsheild pilars's, Tailgate opening, sunvisors, overhead consoles, domelights, coat hooks, grab handles all need to be removed to get the headliner out.
Any holes in the headliner will have to be punched in the new one after its glued on the backerboard.
Helps to have a Repair Manual, so you know what kind of clips and anchors your dealing with when it comes to those plastic brittle moldings that you risk cracking when you try to remove them.
Once all the trim & moldings are removed the backer board the headliner is on is held to the inside of the roof by about 9 or more velcro straps that snap into the roof and the matting end is stapled into the headline backer board. Caution is needed so the straps don't tear out of the headliner. DO NOT REMOVE the velcro straps from the inner roof panel, tear the two velcro halves apart. You could break the tabs off the velcro straps that snap in place in holes in the roof. If any of the straps have pulled out of the roof - silicone sealer can be used to glue them back up. The Velcro strap sometimes dry out to from heat.
Once you get the headliner out you've got to scrap clean the backer board & remove all the old foam.
3M Trim Adhesive is what I used to glue the new headliner in place. Its available at most autoparts stores, K & Wal- Mart have it too. I used almost 3 cans on the car below, with a van you may want to get 5 cans. You can always return them If you don't use them. The trim adhesive has to be applied to both the foam backing of the headliner and the backing board.
Follow the instructions on the can. Its not that hard of a job.
Good Luck !
========= Harryface =========
1991 Pontiac Bonneville LE, 3800 V6 _~_~_~_~274,416 miles_~_~_ ~_~_