Pry off the trim around the door frame on the inside of the room with a small pry bar and hammer. This exposes the rough opening studs and the header above the door. Open the door halfway and ask your helper to hold the door.
Insert a metal cutting blade into your reciprocal saw. Cut through all the nails securing the door frame to the studs and header of the opening. This disconnects the pre-hung door frame from the house framing.
Place a level against the hinge jamb side of the door frame. Insert shims between the house framing studs and the door frame as necessary until the hinge frame jamb is level. Drive 16-penny nails through the frame and into the house studs.
Move over to the door latch side of the door frame and place the level on the jamb. Insert shims as necessary and secure the frame with nails. Drive nails through the header frame. Close the door and check its operation.
Make any adjustments necessary in the door latch to ensure the door knob engages properly. Sometimes it is necessary to loosen the strike plate screws with a Phillips screwdriver and move the strike plate up or down. Tighten the screws to secure the plate. Reattach the trim to the door opening.