I did this 25 years ago. I think it is described in the tech manual but, in short:
- remove gauges from dash
- uncrimp the chrome ring by carefully prying open at the back, with a small screwdriver. No need to uncrimp entire circumference, just enough to remove the ring
- this will allow you to remove the glass
- clean the glass
- installation is by carefully crimping the chrome ring back on, use pliers and protect the chrome with a piece of heavy cloth, a swatch of leather or similar. If you do this properly, you will not be able to see the ring was ever off, from the front
- enjoy the view. Mine are stil clear, now 25 years on