Remove the Door
Remove the door from its hinges by placing a screwdriver under the top lip of the hinge pin and tapping with a hammer to drive the pin out.
Remove the threshold and doorjamb. If the jamb proves difficult, use the reciprocating saw and the metal-cutting blade to cut through the nails, then pry the doorjamb off. Replace any damaged or rotting wood, as it will be against your new doorjamb (this is a situation where you do not know what the situation will be until the old door and jamb are removed.)