Note that it is not necessary to close apps on your mobile phone. The phone will handle it automatically for you.
However, you can close any app on your phone with the following steps:
iPhone/iPad:
- Double press the "Home" button.
- Navigate to the app you want to close.
- Swipe up on the app to close it.
Android
- Tap the "Square" button in the lower right of your phone.
- Navigate to the app you want to close.
- Tap the "X" in the upper right or swipe right to close the app.