The only way to update the phone is via the OTA. OTA means Over The Air and it's a technology which allows phones to update the firmware using only the wireless. Connect to a network and make sure the phone is charged up to 85 % then go to Settings > About phone > Software Update. Register if it asks you then perform the update. While the update process is ongoing, the phone might reboot a few times. This is the way to update your phone's firmware.