Windows Phone 7.8 has been spotted on Lumia 800 devices in India since December last year, but Nokia India announced its official availability only today.
According to a recent tweet from the company, the update has started to arrive on Lumia 800 devices in the country and should hit all devices shortly.
As soon as the update is available for a user, a notification will be sent to the device. However, the update process requires connecting the smartphone to a Windows PC or to a Mac, just as Nokia India explains on this support page.
No specific info on the availability of the update for other Lumia devices has been provided as of now, but a subsequent tweet from Nokia Care did say that it would be rolled out to all devices in phases. Stay tuned for more on this.