So this is interesting, but it seems I now have Denim on my 521, even though the Availability page on Microsoft's website still says that the latest version for the 521 is Cyan.
What I did was to prepare my phone using the XDA hack to update to Windows 10 phones. However, mid-process it said welcome to Lumia Denim!
I had done these steps:
from here: HOWTO: Install Windows 10 for Phones on Non-… | Windows Phone 8 Development and Hacking | XDA Forums
then I realized that my phone did not have enough space, so I reset the phone using the reset button under Settings>> About, when I set it up it had Denim. Note it WAS on the Preview for Developers, but when I reset, I think it went away. So how do I have Denim.
What I did was to prepare my phone using the XDA hack to update to Windows 10 phones. However, mid-process it said welcome to Lumia Denim!
I had done these steps:
4. Download and extract the WPInsidersHacks app (attached to this post) to your computer
5. Start the WPInsiderHacks app on pc, approve any firewall requests
6. Connect to the same WiFi that your PC is on. Edit the settings for the WiFi connection to turn on the Proxy and set it to use the PC?s IP address and enter 8877 for the port
7. Open Internet Explorer on your phone and navigate to http://[pc.ip.add.ress]:8877 where [pc.ip.add.ress] is the IP address of the PC running the WPInsidersHacks app
8. Tap on the link at the bottom to the fiddler root certificate
9. When prompted click open and then install to trust the cert and ok at the confirmation
from here: HOWTO: Install Windows 10 for Phones on Non-… | Windows Phone 8 Development and Hacking | XDA Forums
then I realized that my phone did not have enough space, so I reset the phone using the reset button under Settings>> About, when I set it up it had Denim. Note it WAS on the Preview for Developers, but when I reset, I think it went away. So how do I have Denim.