The Fitbit Community is a gathering place for real people who wish to exchange ideas, solutions, tips, techniques, and insight about the Fitbit products and services they love. By joining our Community, you agree to uphold these guidelines, so please take a moment to look them over.
In the app click on the Temperature tab below the "Health Metrics" on the main screen. Top right you will see a Wheel settings icon. Click on that and you can toggle between C and F.