You can set your phone to use face recognition as phone lock code or to authorise iTunes and App Store purchases.
Press Face ID & Passcode.
Press Set Up Face ID.
Press Get Started and follow the instructions on the screen to set up Face ID.
Press Done. If you haven't previously selected a phone lock code, key in a code of your own choice twice.
Press the indicator next to "iPhone Unlock" to turn the function on or off.
Press the indicator next to "iTunes & App Store" to turn the function on or off.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.