THe ZTE F451 does not have a restart button. You need to turn to manually turn it off then on again.

But if by restart, you mean restore. That will erase all user data permanently and reset all settings to factory defaults. Here's how:

1. Press the Center Select Key [Menu] ➔ Settings & Tools ➔ Phone Settings ➔ Phone Security.
2. Enter the phone lock code and press the Center Select Key.
3. Select Restore Phone and press the Center Select Key.
4. Select Yes and press the Center Select Key.
5. Follow the on-screen prompts and press the Center Select Key to restore the phone.