If you own one of the devices supported by Nokia Belle (previously Symbian Belle), simply connect your device to your computer and start up the Nokia Suite. Click on 'Software Updates' and wait while the application checks Nokia's servers for availability in your region. You will need to update your device using the Nokia Suite as this is a major update which cannot be obtained over-the-air.
Nokia Belle was first announced back in August last year and offers a completely new user interface, an updated notification system and tons of new features.
While the roll-out was officially rumoured to begin on February 8th, the latest version of Symbian is already making it's way to various product codes. Please note that the update may take time to be available to all regions and carriers and may take at least a few days to reach your device.
The folks at Nokia have also provided a step by step video guide showing you how to update your supported Symbian device to Nokia Belle.
source - Nokia Conversations