Whether they're too stiff or too loose or just too worn and outdated, interior doorknobs are something you can easily replace yourself. Here's how.

 
Steps

A new doorknob and its hardware.

Obtain a replacement doorknob. Besides the style, look for something solid that will hold up to regular use and last a long time. Also, consider whether this will go on a bedroom, bathroom or closet. Should it have a privacy lock? If you have lever-style handles and a locking handle, you will need to get a right- or left-handed handle appropriate to the door direction. Buy a doorknob with a pawl the same length as the one you are replacing.

Remove the faceplate screws.

Remove the two screws on the face plate. Twist counterclockwise to remove.

Remove the screws holding the handle.

Remove the two screws on the knob plate. The handles will come loose, so be prepared for both sides to fall.

Remove the handles.

Pull the handles off both sides.

Push out the faceplate and pawl.

Push the faceplate and pawl out through the opening.

Remove the strike plate.
Loosen the two screws on the strike plate and remove the strike plate.

Replace the face plate, if needed.

Some doorknobs come with a choice of face plates. On this one, you can replace the face plate by prying it off with a flat-blade screwdriver. Select the face plate that matches the opening in your door.

This strike plate has square corners for a round-cornered opening.

Size up the new strike plate and pawl mechanisms with the holes in your door and door jamb.

Adjust the openings.

Use a hammer and chisel to adjust the openings in the door and door jamb, if necessary.

Push the new pawl mechanism into the door.

Push the new pawl mechanism and face plate into the door. Be sure the pawl faces the way the door will close, so the slanted side will meet the strike plate. Pushing with your hands should work. Otherwise, you may have to drill the hole a bit larger. Alternatively, you can place a block of wood against the face plate and tap on the block of wood with a hammer. The black plastic collar shown in the photo can help to take up extra diameter if the opening is loose

Screw it in.

Screw down the pawl mechanism, making sure it is as level as possible.

Insert the handles.

Insert the handles into the door so that the square peg fits through the square opening and the screw holes align with the threaded cylinders on the other side. Check that levers and locks face the right directions.

Screw on the handles.

Insert and tighten the screws that hold the handle.

Install the strike plate.

Install the strike plate.

Check that everything fits and closes properly, and make any necessary adjustments.

Tips

A rotary tool, such as a Dremel tool, can be used with an end-cutting bit to shave off excess wood and adjust the slots, if you're careful. A hammer and chisel are usually faster, safer, and more accurate, though.
Read the instructions that come with your replacement doorknob, too. They may have tips specific to that model.
Sitting on a low stool or chair can make it easier to work at this height, especially if you're doing any chiseling. A doorstop or other object to keep the door still can help, too.
Once each pair of screws is snug, try tightening them both a final time. Sometimes one will loosen a bit as the other tightens. Don't over-tighten, though, especially on the wood screws.
If the old wood screws have left mangled wood that will not take a screw, fill those holes with a hard Spackle or car bonder. Wait until dry, drill pilot hole, and place your new screws.
If the old strike plate is okay and matches, you could just leave it be. It's not a part that wears out or gets much attention.

Warnings

Wear safety glasses when working with a chisel or rotary tool. Follow manufacturer's instructions and keep long hair, loose clothing, and fingers clear of a rotary tool.
Proceed slowly when removing wood. It is much easier to remove a little wood than to add some.
Follow basic chisel safety rules: never chisel towards your hand, always use a very sharp chisel, and use the correct sized chisel. Do not use a chisel if you do not know how.
Most interior doors have hollow cores, which mean you will not have much wood to work with. If your pawl is too long, for example, there will be little wood to drill out, so work carefully: measure twice and cut once.
Check to make sure that knobs are on securely and work on both sides before closing the door!

Things You'll Need

Safety glasses, screwdrivers, chisel, hammer.

Replacement doorknob

Flat and Phillips head screwdrivers
Hammer and chisel
Safety glasses
Low chair or stool
Doorstop or heavy object (optional)
Broom and dustpan or vacuum cleaner
Drill and bit, if the opening needs adjustment