Open iBooks and download the free Apple IPhone guide for iOS 11.1 which includes the iPhone X. Go to page 76.
“Switch between apps
While using one app, you can easily switch to another.
View open apps.
iPhone X: Swipe up from the bottom edge and pause to show the App Switcher.
Other models: Double-click the Home button to show the App Switcher.
All of your open apps appear in the App Switcher. Swipe right to see more. To switch to another app, tap it.
Tip: (iPhone X) To quickly switch between open apps, swipe right along the bottom.
Close an app. If an app isn’t working properly, you can force it to close, then try to reopen it. (Typically, there is no reason to close an app; closing it doesn’t save battery power, for example.) To close an app, open the App Switcher, swipe right to find the app, then:”
Excerpt From
iPhone User Guide for iOS 11.1
Apple Inc.
https://itunes.apple.com/us/book/iphone-user-guide-for-ios-11-1/id1263310224?mt= 11
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