Option 1: Combined keys
Simultaneously press the Power and Volume up buttons to call the operation interface up. You'll see four options: Emergency, Lockdown, Power off, and Restart. Click Power off or Restart.
Option 2: Control center panel
Pull down the status bar, slide down on the control center panel, and click the icon in the lower right corner to call the operation interface up. You'll see four options: Emergency, Lockdown, Power off, and Restart. Click Power off or Restart.
Tip:
If your phone doesn't have a crash screen, you can press and hold the power button for 15 seconds to force shutdown, or press and hold the power button for 15 seconds or more to force restart.