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.