When you exchange some or all of the interior knobs, called "lever sets," do a bit of legwork in the nearest hardware store. Lever sets come in both interior and exterior versions, both as single units and in kits of two that contain one interior and one exterior handle. Lever sets, like door knobs, seem to come in a nearly infinite variety of designs. Whether your final selection is ornate or purely functional, the installation technique is the same.
Open the door and brace it in the open position. Remove the two mounting screws from the plate, called the "rose," at the base of the lever set on the interior side of the door.
Hold the external knob with one hand. Slide the interior lever set away from the door with your free hand and set the lever set down.
Inspect the latch mechanism and the "tail" of the exterior knob that slides through the latch for damage.
Slide the new lever set onto the tailpiece of the exterior knob, while holding the exterior knob tight against the exterior surface of the door. Push the lever set toward the interior side of the door until the rose comes into contact with the surface of the door.
Insert the two screws into the mounting holes in the rose of the interior lever set while continuing to hold the exterior knob against the door. When the screws are threaded into the mounting posts of the exterior knob, they will be harder to turn. Use a screwdriver to tighten the screws. Check the operation of the exterior knob and interior lever set.
Rubbing the threads on a piece of soap eases their entry into the mounting posts of the exterior knob.
Do not try to replace an interior handle without opening the door. Unless you hold the exterior knob in place while mounting the interior handle, the interior handle will push the exterior knob out of the latch and the door when you try to slide it onto the tailpiece from the exterior knob.