Members of the Windows Insider program can download a new version of the Insider app for Windows 10 Mobile. However, this download looks like it's a minor new version of the app, with no published change log from Microsoft.
The version number for the Windows Insider app has gone up from 126.96.36.199 to 188.8.131.52, which should give you an indication of how small this update is. The app allows users to sign up to test preview versions of Windows 10 Mobile, with the option of joining either the Fast or Slow ring for updates. You can also provide feedback from the app directly to Microsoft to help improve future versions of the mobile OS.
Thanks to everyone for the tips!