My dash and ~20 screws holding in place placed along the outside edge. I removed each screw and then gently pried the dash out. If yours seems stuck you may have to get/make a tool to get behind the edge and pry it up.
Be careful, there is a good chance you will crack the dash. Also pretty good chance the gauge is obsolete, I know mine are. Do you have a plan on what to do if you can't replace with same gauge??
Personally I don't even use my gauges anymore, I bought a Silverleaf VMSpc monitoring system and all the important parameters are displayed on a small tablet. It is much more responsive then the gauges. I can setup alarms, and also data log any that I want. Works great. There are other options, Scangauge, Silverleaf, and the most recent is Bluefire.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi