Summarize this article:

Now that you’ve set up Do Not Disturb, here’s how you can enable and disable it quickly from the home screen. It’s the fourth icon at the top of the screen. If the moon was previously gray, it will turn white, which means Do Not Disturb mode is enabled. You will not receive incoming calls in this mode. If you don’t see the moon, see Customize the Control Center on iPhone or iPad to learn how to get it. The moon icon will turn gray again, and you’ll be able to receive phone calls.

Summary:
Swipe up from the bottom of the home screen. Tap the moon icon. Tap the moon icon again to toggle off Do Not Disturb mode.