Windows Phone 7.8 update/upgrade has many and whole of new features and looks than older WP 7.5.
It has started from today.
Nokia Lumia has rolled out Windows Phone 7.8 update for Lumia smartphones like Lumia 510, 610, 710, 800 and 900.
To update the OS, connect your lumia to the PC via Zune.
An then follow the instructions to upgrade ur lumia.