You'll need to pull the cluster, then separate the gauges from the front glass. Instructions are at the link below. It's reasonably straightforward to do.
How to fix your slow reading speedometer or tachometer on 91-94 Spiders
Note that the window is glued to the plastic piece that has the surrounds for the speedometer and tach and probably cannot be separated without breaking it (see the first pic in that thread). This limits how much of the rear of the clear plastic you can reach.
Hopefully its just dusty, in which case you can just blow it out. I also had decent luck sloshing around some water with dishwashing soap in it to clean it out and then using anhydrous (0% water) alcohol to flush it so it dried without spotting.
While you have it out, get some Novus or Plast-X plastic polish and give the front side a good polishing - makes a big difference.
Linear Mode
