You can update your phone to Nokia Belle using the Nokia Suite software or using a Wi-Fi connection (is best).
– how to update your phone wirelessly
– Update using Nokia Suite
1.To update your phone, you need Nokia Suite software on your computer.
2. Connect your phone to your computer with a compatible USB cable.
3.Start Nokia Suite, and select Tools > Software updates. Then select the updates you want to download, and select Install.
Video on how to update your phone using Nokia Suite.
Note : recommended that you create a backup before doing a software update using Nokia Suite.
Source : http://www.nokia.com