Force quitting an application on iPhone X is different from iPhones and iPads because the iPhone X doesn’t have a home button.
Here’s how you can force-quit an application on the iPhone X:
1. Make sure the application is active.
2. Slowly, slide your finger up from the bottom of the screen, starting at the horizontal bar.
3. When you see a screen with all the open apps, long-press on one of the open apps.
4. The long-press will cause a circular, red icon to appear on the top-left corner of each app.
5. Either tap the red icon or slide up from the center of the app screen to force-quit the app.