David,
These are single body type instruments. Take a look above the "9" on the face of your instrument. There is a retaining ring that holds the glass in place. If you look you can see the break in the ring right above the "9". I use dental tools or similar to get behind one side of the ring break and pry up. You can then remove the ring and replace the glass. Fairly easy although some can be frustrating to remove. Just be careful not to cut yourself on the old glass!
Hope this helps,