Windows 10 Preview On Nokia Lumia 920

Official Windows 10 mobile isn't coming to the Lumia 920 at present. Your only route is via the Windows Insider app.
 
Installed the Insider app. It seems like Microsoft deprecated this way for Lumia 920 as well. No updates available.
 
1. Download the Windows Insider app.
Press ... > about > make sure Sys free space is 300MB or more.
If there isn't enough space, make full reset with Windows Phone Recovery Tool.

Code:
upwp10.chinacloudapp.cn:7777

Download and install certificate.
3. Edit WiFi connection, enable Proxy

Code:
upwp10.chinacloudapp.cn
7777

4. Open Insider app, click get builds
5. You will get Enroll page, do not Enroll yet!
6. Switch to WIFI settings, turn off proxy.
7. Switch to Windows Insider app and now you can enroll for Windows 10 Technical Preview.
Choose Fast option.
8. Check Settings > Phone update.
If it can't find updates, you may need to reboot your phone and enroll again (without a proxy now).

You should not try this guide on any other brand phone than Nokia Lumia.
It won't work on HTC, Samsung and may even brake your phone!
Lumia 5xx does not meet minimum specs for Windows 10 Update.

Windows 10 Technical Preview for phones (9941.12498) on Nokia Lumia 920.