Press the small hole located near the handle on the interior side of the lock with a small pick and release the interior knob from the spindle. Set the interior knob aside for future use.
Place a key in the exterior side of the knob and unlock the door. Turn the knob to open the door and push down on the retainer clip located on the side of the knob. Pull back slightly and release the knob.
Slide out the cylinder from the inside of the exterior knob and locate the tumbler inside the cylinder. Remove the C-clip holding the tumbler in place by hand and pull out the tumbler. Match up the old tumbler with a new one and slide the new tumbler in its place.
Tighten the C-clamp back down and slide the cylinder back into the exterior knob. Attach the exterior knob back in place to the door and install the interior knob so it locks in with the exterior just like it was installed previously.
Insert a key and test it out on the door to make sure the tumbler is working properly. Lock and unlock the door multiple times to ensure the tumbler is in good shape.