To force close the app on iOS, double tap the home button and swipe up on the app. For devices without a home button, swipe up from the bottom of the screen and hold until the app switcher appears. Then, swipe up on the app.
To force close the app on Android, open the app switcher and swipe up on the app. If the app switcher is not available, open the Settings app, select "Apps", select "Manage Apps", select the app, and select "Force Stop".