If you live in the U.S., the thermometer you have received is pre-programmed to Fahrenheit units. To switch between Celsius and Fahrenheit, you can press and hold the power button when the thermometer is off, this will change units to Celsius instead of Fahrenheit (and back if needed).
You can also read about this in the instructions for the thermometer you received in the package.
If needed, you are also able to change the units of measurement for temperature by opening your app and going to the menu in the top left corner > Settings > Units. By making this switch, it will automatically convert all of your added temperatures to the desired temperature unit.