In order to to do this, you must first be in Biotech Mode, then: Place the thermometer in the wall holder (Model 692 Only). Press and hold the Recall button, while removing the probe from the probe well. Once the probe is removed, release the Recall button. Press and release the Recall button to move sequentially through the various program categories until you see “Units Default” in the upper left hand corner of the screen. Change default temperature mode to Celsius or Fahrenheit by pressing and releasing the C/F button. Replace the probe into the probe well and the device will go into Biotech mode and return to normal operation.