To Replace a Wooden Door Frame, you have to Remove the old door and frame. In general, you should start with taking off the trim or casing around the door. After that, you need to use a circular saw to cut through the nails securing the old door unit to the surrounding frame to make it easy to slide the door out. Then, saw through and remove the framing studs that form the old rough opening for the door. Next, you must Measure the door that will go into the new frame and mark its width on the horizontal boards that form the top as well as bottom of the wall frame.
Forth, you can make marks beyond these width lines on each side at 3/8, 1-7/8 and 3-3/8 inches from the original lines. If possible, you need to mark the height of your new door on nearby wall studs. After that, Use a circular saw to cut a pair of 2-by-4 studs to fit between the horizontal wall frame boards and position them with using the last set of marks you made. Then, Nail them to both boards with driving framing nails diagonally through them on each side. Forth, Cut a pair of 2-by-4 studs to the height of your door and position them next to the studs you just installed.
After that, Nail them to the studs and the top and bottom wall frame boards. Then, Measure the distance between the studs that should be 3-3/4 greater than the width of your doorway. Next, Cut a single 2-by-4 stud to this width, lay it on top of the studs and nail it to both sets of studs. Forth, Measure the distance from the header to the top of the wall frame. After that, Cut a single 2-by-4 stud to this height and put it vertically onto the midpoint of the header.
Then, Nail up through the header to anchor it in place and nail diagonally through it into the top wall frame board. After that, your new door frame is now finished and you can install your door and refinish the area around it.