AFAIK software is signed off by Nokia but timing of distribution to update servers remains a Microsoft prerogative.
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!
This worked like a charm with my Lumia 900 with AT&T. Follow these instructions, they will work. Also, after the first update, there is another Nokia update that installs... Very simple! http://www.wpcentral.com/forcing-windows-phone-78-