Follow these steps to force close all apps on your device:

On Apple devices without a Home Button (iPhone X and later):
  1. From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
  2. Swipe right or left to find the app that you want to close.  
  3. Swipe up on the app's preview to close the app.
  4. Repeat for all apps.

On other iOS devices:

  1. Double-click the Home button
  2. Swipe right or left to find the app that you want to close.  
  3. Swipe up on the app's preview to close the app.
  4. Repeat for all apps.