I know this one!
If it is an old panelled door (usually 4 or 6 panels tongued into place, or held by mouldings, and the stiles and ?mullions? are mortice and tenon jointed) and you have stripped the paint off:
make sure the hinges and frame are well fixed because you will be putting some force on them.
Open the door, and stand on the side of it where the top is leaning away from you, and the bottom is warped towards you
Put your foot firmly on the floor against the foot of the door
Take hold of the top of the door, and pull it hard towards you. You should hear old paint and glue crack, and the door will bend towards you. The old paint and glue will usually hold it in position, though it will spring back a bit when you let go. If it is a heavy, solid, 2-inch door in what was an expensive large house , you will need a lot of strength. If it is a lighter, inch-and-a-half mass produced Victorian door in a small house, it will be easier.
If you bend it too far, go round the other side and pull the top back the other way a bit.
If you have not stripped the paint off where the panels are grooved into place, the old paint may stop it flexing. You will usually see old paint, or bare wood, around some of the edges of the panels where they have moved in their grooves, after you have racked it.