Since iPhone X is missing a Home button to press twice on the app switch, you need to switch apps in a new way. To retrieve the app switch, drag up from the bottom of the screen to about halfway, and then pause until the app thumbnails appear. Then you can browse your launched apps by swiping horizontally and switching to an app by tapping the thumbnail. While in the app switch, you can also force a frozen app: Press a thumbnail to get a red minus button and press that button.
Alternatively, you can skip the app switch completely. Instead, you go right at the very bottom of the screen to switch to the previous app-swiping left switches for the next app. Hope these tips help!