Microsoft has just begun rolling out an updated Windows 10 for phones preview build for Windows Insiders. The new build can be downloaded from those running the existing preview, or those running Windows Phone 8.1 with Denim via the Windows Insider app available on the Windows Store.
This second build of Windows 10 for phones runs on pretty much all Lumia's, minus the 930 and ICON, which are absent due to a bug that displays the UX incorrectly. Microsoft says those two devices will be able to upgrade to the next build releases, however we don't know when that will be.
New features in this build include new, universal apps for Messaging, Mail and more. A load of UI improvements and changes are also apparent, borrowing from what the Windows 10 for desktops preview looks like. Windows 10 is definitely sharing a universal UI across all form-factors, much like Windows 8 but on a larger scale.
Update: Microsoft has posted on the Windows Blog about the new build, including a whole new list of features and fixes.