Microsoft had mentioned that the first generation Windows Phones will be receiving the Windows Phone 7.8 update since they are not compatible with WP8. Today, Nokia has started the official roll out of WP7.8 update for its Lumia phones. The Windows Phone 7.8 update is beginning its rollout to phones currently running on Windows Phone 7.5. You can transform your Nokia Lumia with a host of exciting new personalisation features, such as the ability to resize your tiles, new lock screen options, and more theme colours although you won’t get the complete list of features available in WP8.
The WP7.5 update notifications will begin to appear for Nokia Lumia 510, 610, 710, 800 and 900 owners with an unlocked phone or operator approved software during February. You can simply connect your phone to your computer via Zune for PC, or the Windows Phone app for Mac. Check this below video for more details.
More details here.