Instruction I:
1. First, press'n'hold the Power button until a menu will appear.
2. Second, select the Power off option.
3. Wait for a few seconds and then press'n'hold the Power button until the phone turns back on. Done!
Instruction II:
1. First of all, take off the phone's back cover and remove the battery.
2. Wait a few seconds and reassemble the phone. Lastly, press the Power button to turn it on. Done!