Open the door and search for the screws that hold the doorknob in place. Generally, these are on the inside of the door where the lock engages.
Remove any screws with the appropriate flat head or Phillips screwdriver.
Inspect the doorknob itself. Often, there are additional screws on either side or underneath the doorknob. Remove these as well to release the doorknob.
Pop off the doorknob by looking for a slot around the edge if the screws don't totally release it. The slot is large enough for you to insert a flat head screwdriver that enables you to pop it off. Normally, additional screws are underneath this area.
Twist out any additional screws in the strike plate or faceplate and underneath the doorknob with a screwdriver.
Look at the opening that is on the area where you removed the strike plate or faceplate. Check to see if it has a groove that is square or round. This is called the latch and you must purchase a knob that works with the shape of this latch.
Stick a tape measure into the opening once all the doorknob parts are out. Measure the area from the edge of the door to the middle of the area with the hole. This is the bore hole that is drilled into the area where you removed the faceplate. This is known as the backset, and this distance is crucial to installing the right size doorknob. Take the measurement with you to purchase a new doorknob.