This week, Microsoft outlined a path for unsupported Windows 10 Mobile phones, unable to run the Production version of the Creators Update, to be able to continue getting Cumulative Updates for their Windows phones. Now today, a new post on the Feedback Hub outlines how to proceed (Windows 10 Feedback Hub required for viewing).
If you're on a supported device and are continuing on the Windows Insider Release Preview Ring, you should be seeing a new cumulative update build 15063.251, if you haven't already updated.
For unsupported devices, there are a few more hoops to jump through. If you've already moved to the Creators Update via Windows Insider:
If you are on an unsupported device and already on the Windows 10 Creators Update (Build 15063): Go to Settings > Update & security > Windows Insider Program and move to the Release Preview ring in order to continue receiving Cumulative Updates for Build 15063. Your device should pick up Build 15063.251 today.
If you're not on build 15063, join the Windows Insider Program in either the Fast or the Slow Ring, and your device will make a number of build "hops" and eventually get to build 15063. Once it's there, then move your device to the Release Preview Ring, and you'll then receive 15063.251 and future Cumulative Updates.
If you want to return your phone to a supported state, you'll have to use the Windows Device Recovery Tool, which will bring you back to the Anniversary Update version 14393.x. From there, you'll continue to receive Anniversary Update Cumulative Updates and your phone will be in a supported state, for example if you need warranty help etc.
One last thing, if you have your device in the Anniversary Update, but in the Windows Insider Program in the Release Preview Ring, but your device is now unsupported to move forward to the Creators Update, you won't be able to receive any Cumulative Updates moving forward. For that, you'll have to revert your device back to Production by going to Settings>Update & security>Windows Insider Program and stopping Windows Insider builds.
It's all a bit cumbersome, but to be honest admirable of the Windows Insider team to provide a way forward for these unsupported devices, even though they're not officially able to run the Windows 10 Mobile Creators Update.