This is easy when you know how
(you don't say) but I'll guess this is a softwood panelled door, probably with 4 or 6 panels, and is about 40mm thick. And that it has been painted numerous times and the joints are full of old paint.
Open the door and position yourself so that you want the top to come towards you.
Put your foot against the bottom of the door, at the edge, on the lock side. Grab the top corner, at the lock side. Pull. It will make a loud cracking noise (this is just the paint in the joints cracking). Pull it a couple of inches. Try it against the frame. If it is not moved enough, do it again and pull it harder. If it has moved too far, go round to the other side and pull the top a bit more gently. You have to pull it further than you want as it will partly spring back when you let go.
When you have got it right, it will tend to stay in place because the panels will be jammed into the stiles.
If it is a poor-quality door where the panels are retained by pinned-on ,mouldings rather than machined grooves, some of the mouldings might come loose. You can sand then clean before refixing them. Nail then only to the stiles, not the panels (panelled doors can move as the wood expands and contracts with the weather)
Have a look at the tenons at the sides of the door, if any are loose you can move on to refixing them, but this is a bit more of a job then just re-shaping a panelled door.
If it is an expensive, thick door in a high-quality house, it will need a lot more effort and you may need an assistant.
