1. To do this, you first have to turn your thermometer “off”.
2. Then, you have to press the “scan” button for 4 to 5 seconds.
3. By doing so, the thermometer will automatically be “switched on” and on the right side of thermometer, you will see F or C in blinking mode.
4. As soon as you see it blinking, you have to press the scan button again.
5. If your thermometer is set to Fahrenheit, then it will change to Celsius. If it is set to Celsius, then it will change to Fahrenheit
You can refer the article as well.