To change from calories to kilojoules from the site, click on "Generator Settings" in the left sidebar menu, then you can select either "Calories" or "Kilojoules" next to the "Preferred unit of energy" section:

 

 

 

From the mobile app, you can tap "Preferences" in the bottom right of the screen, and then select "Generator Settings" to get to the page where you can change these settings: