Below are instructions for terminating apps on different operating systems. Terminating apps should not be confused with uninstalling apps, which could lead to data loss. Terminating an app simply forces the app to start it's lifecycle again and can resolve some issues you may have.
iOS devices without a Home button
Swipe up from the Home Indicator at the bottom of the screen, to midway up the screen.
Release as you see the app switcher appear.
Find the app you wish to terminate and swipe up on the screen shot.
iOS devices with a Home button
Double tap the Home button.
Find the app you wish to terminate and swipe up on the screen shot.
Apple Watch
Open the app you wish to terminate.
Press and hold the Side Button until the power-off slider appears.
Press and hold the Digital Crown until the app disappears.
Android devices
Tap the Overview button (▢) to view your recent apps.
Find the app you wish to terminate and swipe it off the screen to the left or right.