There is a lot of free information on how to fix a sagging door on the internet, so I won’t reinvent the wheel here. I’ll just tell you why you might want to fix this problem. It may not be obvious that a door is sagging. The symptoms may be that it is hard to lock or unlock the door, or that the door doesn’t stay shut because the doorknob no longer latches. It may also be difficult to shut the door because the door runs into the doorjamb. When the door is open there may be paint missing near the top on the side of the door. The problem may be alleviated by lifting up on the doorknob while opening and closing or locking and unlocking the door.
To fix this yourself, the first thing to examine is if the hinge screws are loose. Open the door and have somebody pull up on the doorknob. Try tightening the hinge screws while the door is being lifted. If the screws tighten then try shutting the door and see if the problem is fixed. If the screw just spins in the hole, you need to use a longer screw or a thicker screw.
If tightening screws doesn’t fix the problem then you may have bigger issues, like a door frame that is skewed for some reason. Maybe the foundation of the house settled or the door expanded due to moisture. If it gets harder to use your door in the winter there is a good chance that the door is absorbing moisture and should be sealed and painted during the next dry spell.
You can also use a plane to remove extra wood from the part of the door that is rubbing. Exercise caution. Doors are expensive and if you do this wrong it might look terrible and introduce cold drafts to your home. Before you plane your door try switching the top hinge with the bottom hinge. The top hinge usually bends more because there’s more leverage on it. I’ve done this many times with great success…