A new build of Windows 10 Technical Preview for phones has just been released.
Build 10051 updates the build 9941 released in February and makes some major app updates: the new Project Spartan browser is included (though in this build, Internet Explorer 11 is still available and the default browser), as are the new Outlook-branded Mail and Calendar apps. The Phone and Messaging apps have also been replaced with new universal apps that were first shown off in January.
The new build also greatly increases the number of phone models supported. Most Lumia-branded Windows Phone devices can be used with the new build, including high-end models such as the Lumia 1520. There are a few exceptions, however; the Lumia 930, Icon, and new Lumia 640 XL are all unsupported, apparently due to an issue with interface scaling that's still being addressed. Non-Lumia models are also unsupported.
In common with other recent updates, the new build is initially being pushed out to people on the fast update cycle. Devices running 9949 should show that an update is available soon; devices running Windows Phone 8.1 should use the Insider App to kick off the update process. Though right now it appears that Microsoft has some issues with its servers that are preventing phones from updating.