The update is available for download via Zune desktop software
As expected, Nokia and Microsoft have started the rollout of the Windows Phone 7.8 update. The first smartphone to receive the upgrade is the Lumia 800, but other Lumia handsets might be eligible for Windows Phone 7.8 as well.The folks over at WP7.nl report that Windows Phone 7.8 is now available for download via Zune desktop software, so Lumia 800 owners might want to connect their smartphones to a PC and check for the update.
It appears that the major upgrade has been split into four smaller updates, 7.10.8773, 7.10.8779.8, 7.10.8783 and 7.10.8858.136, which will be downloaded and installed on the Lumia 800 in this order.
For the time being, the Lumia 800 is the only one receiving the Windows Phone 7.8 upgrade, but those who own other WP7.5-powered devices will surely get it as well within the next few days.