The Windows Phone 7.8 update for first generation WP phones was neglected by Microsoft for a while, but the company committed to an early 2013 release and now builds for the WP7 Lumias have started to appear on Navifirm.The firmwares are version ****.****.8858.***** and are available for the entire first-gen lineup - that is the Lumia 510, 610, 710, 800 and 900. They can be downloaded and installed using the Navifirm tool.
We should note that these are not official firmware updates, so if
you choose to install them you will be doing so at your own risk. With
the end of 2012 in sight, you might be better off just waiting for the
official update to hit.
If you're not clear on what changes the Windows Phone 7.8 update will
bring (Microsoft was mum on this for a long time), you can check out this Lumia 900 that runs on the updated firmware.