You will need to replaces the door pins & bushings for that door. It is fairly simple. You will need for someone to hold the door and then remove the pins that hold the door hinges together and then replace with new ones. You might be able to just replace the bushings but if your door has been like that for a while then you will probably have to replace pins and bushings. Check the door striker also to see if it needs to be replaced because when the door gets hard to shut it will destroy the striker. This should fix the sagging door. Let me know if I was of any help.