Please follow this guide;
1. Just download this Software :
NOKIA UPDATER2. after downloading, install it on your computer,then run it
3. be sure battery is fully charge and you need an internet stable connection to avoid interruption.
4. plug your phone using a nokia usb data cable,some drivers are installing and wait until the phone is recognized.
5. just follow the procedure to begin with.
6. wait until the update is done...
Hoping this will help you...good luck