It was the maligned “Mid-Range Flagship” on AT&T that was going to get its Windows 10 Mobile update someday. Long after everyone else has left the station, — and the device left Microsoft’s support — Big Blue has moved upon the Nokia Lumia 830.
In a support forums post, it has announced the milestone Windows 10 Mobile update for the device. As of this post, the update has not been officially acknowledged in AT&T’s Device Support section.
That said, you should know what to do at this point: download the Upgrade Advisor app, check for update availability and then make you have at least 1GB of space free. That’s how much space this package takes.