I know this is an old post, but I thought add to it just in case someone else has one of these gauges that is reading inaccurately and wants to fix it. Mine was reading about 20 degrees high.
I un-crimped the bezel from the gauge body and removed the glass lens to expose the gauge face and indicator needle. I gently pried the needle off its shaft with a small screwdriver, put the gauge's sensor next to a digital thermometer, then pushed the needle back on at the correct temp. The hardest part was re-crimping the bezel to the gauge body - not pretty but it's hidden when mounted in the gauge pod.