Open Control Center
Tap the Flashlight icon to turn it off
If the flashlight is on from the Lock Screen, press and hold the flashlight icon in Control Center, then tap it again to turn it off
If using the Camera app, tap the Flash icon until it is set to Off, or switch off the torch option if shown
Swipe up from the bottom of the screen (or down from the top-right on iPhone models with Face ID) to access Control Center, then tap the Flashlight icon again