Apps that run in the background are applications you've turned off, but still run in the background. Here I will guide you how to delete running apps on the iPhone.
Method 1: Use the Home key
Step 1: Press the Home key twice to reveal the multitasking.
Step 2: Swipe up to delete the application running as you wanted.
Method 2: Use 3D Touch
With the iPhone Touch 3D on iPhone 6S, 7. You press lightly on the edge of the iPhone and drag across as shown to the multitasking. Then delete the application running in the background.
Method 3: Swipe (iPhone X only)
First you swipe from the toolbar on the bottom and keep a while.
Immediately the multitasker will pop up and you can delete applications running on the iPhone X.
If you want to go to another app just tap on the app's icon. To close an app, simply swipe upward on that app's thumbnail until you flick it off of the screen. You can close just one app, or close them all if you like. It is just an easy step. Hope you get it.