1. Turn off the phone by holding the Power button.
2. Press and hold together Power + Volume Up button for several seconds
3. From the menu on your screen choose "RECOVERY" by using Volume Up to scroll andVolume Down to select.
4. Afterwards press the Power button to enter Recovery Mode.
5. Press Volume Down to choose "wipe data/factory reset", and then push the Volume Up button to confirm.
6. Select "Yes--delete all user data". Use Volume Down button to scroll and Volume Up to select.
7. To restart the phone, select "reboot system now".
8. Success! The hard reset is done!

Second method:

1. Push the Power button to turn on the device.
2. Go to Menu and choose Settings .
3. After that tap Backup&Reset and select Factory data reset.
4. Then choose Reset phone, and confirm the whole operation tapping Erase everything.
5. Well done!