First method:

1. At the very beginning hold down the Power key in order to switch off the cell phone.
2. Then hold and press the Volume Up + Power button for a couple of seconds.
3. You can releas held keys when the phone’s logo appears on the screen.
4. Select using Volume buttons option “wipe data/factory reset“, and confirm that with Power rocker.
5. In the next step choose “yes” from the menu, and accept it with Power key.
6. Now choose option “reboot system now” by using Power button.
7. Success! The hard reset has just been performed.

Second method:

1. If the display is off, press the Power key to turn it back on.
2. Afterwards choose from menu Settings  and tap Backup & Reset (Privacy).
3. Then choose Factory data reset and select Reset phone.
4. Confirm all informations, and wait until your phone will reset and reboot itself.
5. Well done! Your phone is ready to use.