An ECG rhythm will appear upside-down if the mobile device is not properly oriented while the data is being acquired. For proper orientation, see From where can I take a reading? How do I orient the monitor? How much pressure do I apply?
You may invert an ECG that has previously been recorded by tapping the screen while reviewing the ECG in the Kardia app, and tapping the 'Invert' button that appears in the bottom right corner.