Swipe up from the bottom of the screen and pause in the middle
Swipe left or right to find the app
Swipe the app card up to close it
On iPhone with a Home button, double-click the Home button
Swipe left or right to find the app
Swipe the app card up to close it
If the app is frozen, restart the iPhone
If needed, go to Settings > General > Shut Down, then turn the iPhone back on