Here's how to remove the clear plastic finger wheel (what most people call the dial) on most Western Electric, Stromberg-Carlson, Comdial, Cortelco, ITT, Northern Telecom or Northern Electric phones.
Look between the "9" and "0" holes on your finger wheel, and find the tiny hole.
Put the tip of a paper clip into that hole. Sometimes a push-pin is easier to use.
Rotate the finger wheel all the way clockwise and push down on the paper clip or push pin.
This will release the spring clip that holds the finger wheel in place.
While holding the spring down by pushing on the paper clip or push pin, force the dial one more space clockwise and the finger wheel will come loose.
You can now remove, write on, type on, stick a label on, or replace the round designation card that fits in the clear center of the finger wheel. Some of the cards have a perforated notch that should be aligned with a point molded into the finger wheel.
To put the finger wheel back on, put the "0" hole over the numeral "9" and turn the wheel counter-clockwise. The spring will jump back into the slot and the finger wheel will stay in the proper position.
The procedure is even simpler with old metal dials. Simply press the tip of a small, flat screwdriver blade (about 1/16th of an inch) against the tab built into the dial ring between the number two and number three on the dial, and the metal ring should pop out.
Remove the transparent plastic disc, remove the number label if any, insert the new label, re-insert the clear plastic, and then snap in the metal ring, from the bottom first. (You may find one or two metal pieces under the clear plastic, but they should be easy to remove and replace.)