Here is the way:-
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.