Windows 10 Technical preview was released for existing Windows Phones and you can install the preview or beta version of the new Windows 10 on your Lumia smartphones. Here is the detailed video on the steps. It will be available on Lumia 630, 635, 636, 638, Lumia 730, Lumia 830 and in future will come to Lumia 520, Lumia 525, Lumia530, Lumia 535, Lumia 435, Lumia 532, Lumia 920, Lumia820, Lumia 1320, Lumia 930 and Lumia 1520.
Join Insider program- https://insider.windows.com/
Open the link and sign up using your Microsoft ID.
Download Windows Insider App- http://www.windowsphone.com/en-us/store/app/windows-insider/ed2b1421-6414-4544-bd8d-06d58ee402a5