To flash your nokia you will need to latest flash files of your phone you can download it from here.
This Nokia classic mobile phone comes in the BB5 group. Other groups are DCT3, WD2 and DCT4. It contains mcu, ppm and cnt.
You can flash your mobile phone with flashing software phoenix service. Before flash don’t forget to take backup of your important data. After flashing you will lose your all of data and your mobile will restored in its original settings.
After successful completing of flashing mobile will be restarted manually.
Nokia 6303 Flash File