We do factory calibration all of our Micro Voltage Meters 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 removing the 4 screws from the housing and turning the adjustment screw on the circuit board 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.