You may want to read these posts first:
How to install Windows 10 Technical Preview for Phone (Lumia 630, 635, 730 and 830 only)
The new features in Windows 10 Technical Preview for phone
How to install Windows 10 Technical Preview for Lumia 730, 630 and others

When Windows Phone 10 Preview was released, it was restricted to only couple of devices, but now a little complex hack has got this sorted. Right now only tested on AT&T Lumia 1520, it allows you to install the preview version on any phone. You will need to test it out with your phone. We recommend you do it only on secondary phone as there is not many features to enjoy right now.

How to install Windows 10 for Phone on Lumia 920
Lumia 920 Windows 10 for Phone Installation

Steve T-S on twitter was able to install this on his Lumia 920, adding weight o the point that it can be installed on other phones as well.
Keeping off the complexities, the installation happens by fooling the server with set of data that allows the update. The preview app only looks for Phone Manufacturer Name to decide if the preview if offered and this is where the hacker, RustyGrom, had its success. Here is how you do it.
  • For the Lumia 1520 (and other phones?) reset your phone using the Windows Phone recovery tool. I recommend not logging in and setting everything up yet and just waiting til the end and doing a reset and letting it restore after you’re on Windows 10. This may not be required for your device but most likely is due to the space issue cited by Microsoft. You can see your available space in the About menu of the Windows Insiders app. Before resetting mine was ~200mb and after it was ~600mb. You can try without resetting but may get an error after it tries to download and install the update.
  • Install the Windows Insider app on your phone (you will need to have a store account configured)
  • Open the insiders app and go to the about options in the app bar and screenshot/make notes/email the settings to yourself. You may need this later.
  • Download and extract the WPInsidersHacks app (attached to this post) to your computer
  • Start the WPInsiderHacks app on pc, approve any firewall requests
  • 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
  • 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
  • Tap on the link at the bottom to the fiddler root certificate
  • When prompted click open and then install to trust the cert and ok at the confirmation
  • Run the Windows Insider app and tap get preview builds
  • Tap on which custom action you’d like to perform. In my case I hit “Set to ATT Lumia 635″ and click the arrow at the bottom
  • Accept the agreement and click the check mark at the bottom. the app will close
  • Now clear your proxy settings and run the insider app again
  • Tap get preview builds and login with your Microsoft account
  • Select the fast branch (if you’re reading this of course you want the fast branch) and click the arrow at the bottom
  • Go into the phone settings and check for updates. With any luck you should have Windows 10 there for you to install. On my AT&T 1520 it had to do 8.1 first but it kept going to 10 after 8.1 installed (had to check for updates again)
  • 17. You’ll probably want to go into the settings and do another reset and then log into the phone and let it restore your previous data
That said, do it at your own risk. If anything goes wrong, nobody will be responsible.
Download the files from the XDA Forum.

<Due to copyright claim, we’ve removed the link, you can search Google for “This Hack Lets You Install Windows 10 on Any Windows Phone”>

See also:
How to install Windows 10 Technical Preview for Phone (Lumia 630, 635, 730 and 830 only)
The new features in Windows 10 Technical Preview for phone