Where is it sagging. The headliner is held in place by a combination of staples and glue. It's stapled around the rear window, side windows and windshield. It's glued around the sunroof opening. Most likely the glue has come loose. If the headliner is in good condition it can be reglued using 3M weatherstrip adhesive or contact cement. I recommend using a hair dryer to soften up the vinyl to make it easier to work with.