1. First of all, press'n'hold the Power button.
2. A new window will appear. Drag it down the display, which will turn the phone off.
3. After the phone turns off, wait for fifteen seconds and power it back on by holding down the Power button again.
1. This one is short. Press & hold the Power and Volume Down buttons at the same time. Wait until the phone vibrates. When it does, release held buttons. Voila, done.