There's several differant ways to fix it.
The simplest way but not the best way would be to cut a tapered shim, that's been primed and painted on all sides and attach it to the bottom of the door.
The best way would be to install an adjustabe threshold and add a seal to the bottom of the door.
A lot more work and you would have to have an ossilating saw to be able to get the sides to fit properly. But it can be done.
You also would have to cut the door shorter inorder to fit the new threshold.