Press and hold the side button and either volume button until the power off slider appears
Drag the slider to turn off the iPhone
If the screen is frozen, press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side button until the Apple logo appears and the device restarts
For iPhone SE, iPhone 8, or earlier, press and hold the top or side button until the power off slider appears
Drag the slider to turn off the iPhone