Unscrew the hinges from the door frame using a screwdriver. Typically, doors have three hinges, one at the top, middle and bottom. Each hinge has two or three screws.
Pull the door away from the frame. Turn it around so that if the hinges were on the left edge of the door, they are now on the right edge.
Align the door into mirror-image position. Make marks where the hinges should go using a pencil.
Screw in the hinges at the marks using a screwdriver. Swing the door open and shut a couple of times to make sure it's properly aligned.
Patch the old screw holes with wood putty. Sand down with sandpaper until even with the surrounding surface. Paint over the holes.