How to remove app from memory
Removing an app from the memory means making it stop running in the background.
Here is how you can do it (steps taken from Mac Rumors):
1. At the Home screen of the iPhone, or while in an app, swipe up from the bottom of the screen and pause while still pressing the screen.
2. When the App Switcher comes up, swipe left and right through the different app cards to find the app that you want to close.
3. Use a quick swipe upwards to close the app.