Microsoft uses the same "Windows Insider" program that they use on the PC. I'm been running Windows 10 technical preview on my Lenovo X200 laptop since it was released last fall.
Windows Phone 10 technical preview is easily installed via the new Windows Insider mobile app. The app is available via the Windows phone app store. However, at present, there are a limited number of Windows Phones that support the preview: Lumia 630, Lumia 635, Lumia 636, Lumia 638, Lumia 730, Lumia 830. Support for additional phones is coming. If you don't see the app in the catalog then your phone can't be upgraded at this time. Upgrading involves just clicking a few buttons and a few reboots of the phone. No PC required! New features:
- Full-size Background Image for Start: We believe phones should be intensely personal – so we’ve added another option to customize the start screen with a full-size background image.
- More Quick Actions in Action Center: We’ve increased the number of quick actions available to you. Windows Phone 8.1 has four programmable quick actions – but with Windows 10 you also get an expanded view that can now have up to three rows.
- Interactive Notifications: Notifications are now interactive, allowing you to take action directly like dismissing an alarm, or seeing images for maps. For example, for text messages, when the toast pops, you can quickly reply inline via text or voice.
- Significantly enhanced speech-to-text capability: You can talk to virtually any data field you choose. Your words show up as you speak them – and punctuation appears automatically. This feature is smart enough to understand when to use ‘two’ – the number — instead of ‘too’ as in ‘also’.
- More powerful Photos app: From your very first launch, Photos will show the aggregated set of all your local photos and all your OneDrive photos
More details on Microsoft's blog. Microsoft is expected to release regular updates with new features prior to the anticipated release in the September, 2015 timeframe.