Open the Settings app
Tap Control Center
Add Flashlight if it is not already included
Go to the Lock Screen
Swipe down from the top-right corner on Face ID iPhones
Swipe up from the bottom on Touch ID iPhones
Tap the Flashlight icon to turn it on
Press and hold the Flashlight icon if needed on some models
If Flashlight is missing, restart the iPhone
Update iPhone to the latest iOS version