Doorknobs are fairly simple mechanical devices found in just about every home, unless you live in a barn. In older homes built before World War II, most every knob was connected to a thin metal box that was carefully fit into the edge of the door. These full-mortise lock sets require great skill to install. Replacing them is hard because parts are tough to locate.
In the 1950s, a transition was made to what is now known as the tubular lock set or doorknob. This lock set-and-knob combination required two simple holes to be drilled into a door. The hole sizes, for the most part, have been standardized over the decades, and replacing a tubular doorknob and lock set can be quite simple.
Inspect both sides of your existing doorknob and door. Look for screws, slots, small buttons, etc. at the base of the knob or handle. Look for screws on the faceplates of any metal trim plates that are on the face of the door. Inspect the edges of these plates for small clips or detents. Discovering these things will help you start to disassemble the existing doorknob.
The most basic tubular doorknobs and lock sets often have two screws on one side of the doorknob assembly. These screws will be on a round faceplate on the face of the door. Remove these screws if you see them. The doorknob on this side of the door should slide off to reveal the inside workings of the lock set as well as the large hole drilled in the door.
If you don't see screws on the faceplate, small spring-loaded slots on the knob handle may allow you to pull off the knob handle once depressed. With the knob out of the way, you can then remove the round decorative faceplate. This plate may be held in place by a small spring clip or metal detents in the rim of the faceplate. Use a wide, flat, stiff putty knife to gently pry off this faceplate.
With one side of the knob off, you can determine the size of the hole drilled in the door. Measure the diameter of this hole to ensure the new knob you'll buy will fit in the door.
Hook your tape measure on the edge of the door, and measure to the top or bottom center of the large hole. You should get a measurement of 23 / 8 or 23 / 4 inches. This is called the backset. Be sure the doorknob you plan to buy will work for the measurement you have. Most modern doorknobs will adjust to either dimension.
Stop now and take photographs of what everything looks like, in case you have to reassemble the knob. Look for other screws under the decorative plate you've removed that may pass through a large round metal plate. Unscrew these fasteners to gain access to the inner workings of the doorknob. Take additional photos as you remove more parts.
Remove the other side of the doorknob. It usually just slides out once all screws are removed that hold the two halves together.
With the knob out of the door, remove the spring-loaded latch that extends from the edge of the door into the center of the large hole. Two screws on the edge of the door allow you to remove this hardware. A decorative plate on the edge of the door may have to be removed first, allowing you access to two hidden screws.
Purchase a new doorknob that will fit in the predrilled holes you now see.
Follow the instructions with the new doorknob to install it. Usually you follow the same methods above, just do it in reverse order.
Summary The most important part of replacing a doorknob is making sure the new one will fit into the existing holes and slots you have in your existing door. Take the time to figure this out before buying a new doorknob.