In order to get to your Settings, please click on Profile on the bottom bar of the app.

Then on the next page click on the gear symbol - there are your Settings.

Then click on Personal Details. To change your calories per day goal, tap on Calories / day and overwrite the given value. Save the new number by tapping on 'Save'.

Alternatively, you can, of course, always change your goal by going to Settings then to My Goal. There you can start the goal setup with all details from the beginning.