Loose door sill

I noticed today that the rubber inside part of the rear passenger door is coming down. The car is a 01 Regal GS. Does this sill attach the metal it goes around or does it just slide on it. It is coming down around the center of the door and just looks bad. I mean it's not hanging down just loose and lower than it should be. When I push it back up, it just falls back down. Any ideas here?