Warning! Do this at your own risk! All your phone data (contacts,photos,videos,music,messages,aplications,games,files,) will be lost!
Please make backup first if you can!
1.First power off your phone
2.Press together volume up + home key + power button
3.Browse with volume keys and select "wipe data/factory reset" and then press home key to select it.
4.Browse with volume keys and select "yes -delete all data" and press home key to confirm
5.Wait until the format ends
6.Select "reboot system now" and press home key to confirm
7.Wait until the phone restarts
That's all!
For more details watch HERE the video tutorial.