"Performing a factory data reset deletes all data, including downloaded applications, and resets the device back to its initial state—the state before you turned on the device for the first time.
Important Make sure to back up important data you have on the device before you perform a factory data reset."
1. With the phone off, hold the Volume Down button and press and release the Power button.
2. You'll boot into the menu you see above with the little skateboard guys. Select Clear Storage from the list by pressing the Volume Down button.
3. Press the Power button, and confirm by pressing Volume Up.
4. Sit back while your phone reboots in its virgin state.