We do factory calibration all of our Micro Voltmeters as part of the assembly process, but it is possible for the readout to drift over time.
To calibrate your Micro Voltage Meter, you should first check your power supply's voltage with a recently factory-calibrated multimeter (we recommend Fluke multimeters). Once you have an accurate reading, you can calibrate the voltage readout of the Micro Voltage Meter to match by turning the adjustment screw located in the center of the back enclosure panel with a #0 Phillips head screwdriver.
Be careful not to strip the housing when reinstalling the screws. The screws should only be tightened until snug.