Windows 10 is a unified platform that supports not only desktop computers but also tablets, as well as smart phones as well. If you have one of the supported Windows Phones listed here, you can test out the next version of Windows Phone platform now.
I happen to have a Lumia 520 phone that I was able to put Windows 10 Mobile Technical Preview on it. And here is the steps I took to get there.
First of all, make sure your phone is running on Windows Phone 8.1. Upgrade if it isn’t. My phone was running Windows 8 and I was having difficulties to upgrade it to 8.1. What I ended up doing is to reset the phone to factory default and was able to upgrade to 8.1 afterwards.
Once your phone is running on Windows Phone 8.1, go to Settings to install all pending updates until your phone is running the latest with no any patches leaving behind.
Now open Store app, search for Windows Insider app and install it on your phone.
Launch Windows Insider app, and follow the instruction on the screen to finish the initial setup. Note that you will need to sign in either a Microsoft Account or a domain account to be able to continue on. Windows 10 Mobile Technical Preview also has 2 update options, Fast ring to get updates as soon as they are available, and Slow ring to get a more stable update. You can pick the update option from Windows Inside App.
Then wait until your phone reboots, go to Settings → Update again, Windows 10 Mobile build should be now waiting there to be installed. The size installation is about 1.5 GB so it may take quite a while to finish.
Once the installation is done, go to Settings → System → About to verify the installation.
The further update will be downloaded and installed automatically as soon as they become available. But you can always go to Settings → Update & Recovery → Phone update to manually check as well.
The Windows 10 Mobile Technical Preview has a lot of exciting new features. If you happen to have one Windows Phone laying around, you definitely should check it out.