For which models. My question is specifically for Lumia 1520, which started 2 weeks ago.
June 23, 2016 to be more specific.
AT&T's Lumia 1520 is finally getting the Windows 10 Mobile official update | Windows Central
windows 10 mobile was made available early this year for all eligible phone models, which included 1520, irrespective of which country you were from.
There were conditions such as your phone had to be on denim before updating
Second was the upgrade would only come if you downloaded upgrade advisor app and used that to check - this actually made your phone ready to receive the upgrade
So if your download doesn't work for the app - try a few times.
If it still fails - check back with us, we'll show you how to use insider program
To use insider programc - enrol into windows insider program
Download insider app
Install app, sign in, select release ring and follow instructions
before updating, make sure auto update of time is off and the time, date, time zone and region settings are correct