Since the iPhone X lacks a Home button to press twice for the app switcher, you’ll need to switch apps in a new way. To bring up the app switcher, swipe up from the bottom of the screen to about halfway, and then pause until the app thumbnails appear. Then you can scroll through your launched apps by swiping horizontally and switch to an app by tapping its thumbnail. While in the app switcher, you can also force-quit a frozen app: press a thumbnail to get a red minus button and tap that button. Alternatively, you can skip the app switcher entirely. Instead, swipe right on the very bottom of the screen to switch to the previous app—swiping left switches to the next app.
