From the Microsoft article:
Hi everyone, we thought we would end the week with another build for you – but this time it’s for mobile. Build 10166 of the Windows 10 Mobile Insider Preview is rolling out right now to Windows Insiders on the Fast ring. There are lots of small changes and feature tweaks in this build, but our major focus over the past few builds on mobile has been about improvements to core quality.
Most of the known issues we listed for Build 10149 have been fixed. We also fixed the issue where the post-upgrade migration status screen was missing, which made the Lock screen appear frozen without the date and time for up to 10 minutes. The post-upgrade migration status screen should appear normally now. We also fixed the issue where you were unable to unlock your phone after entering your PIN and it keeps letting you type even though you entered your pin correctly that was caused by a TPM issue. The colors in apps should now appear correctly including in including the titlebar in the Outlook Mail app which is no longer orange. And we also improved reliability of app downloads and updates from the Store (which is now no longer marked as “Beta” and the old Store has been removed).
We also got a lot of feedback about the size of the battery icon, so we bumped that up a bit larger to make it easier for everyone to see.
Be sure to check the Store and grab the latest app updates from there too, as there have been lots of improvements to apps over the past few weeks as well.
If you don't see the update and you're on the programme, try going back into the Windows Insiders application (which you may need to install again) and going through its set-up sequence again.
As usual, there's the dilemma as to how to handle the update. For previous builds the thinking was that a factory reset after the update, following by rebuilding things manually, with data syncs and Store installs, was the way to go to minimise glitches. And I write that from bitter experience from four devices now on the Insiders programme.
However, with bugs being quoshed each time and with the new Store now seemingly behaving, I think it's time to backup using the built-in Windows 10 mechanisms, just in case, and then just let the over the air update take place.
If all is well then you're good to go, if it's not then factory reset and then restore from your backup. Either way, you shouldn't have to rebuild everything from scratch again. Your comments and data points welcome!
Post-upgrade, my Windows Insider devices seem to be fine, with the caveat that your first trip into the Store may result in a lengthy wait while up to 20 or so application updates complete. Please do allow for this. The good news is that more of the provisioned ex-Nokia applications for Lumia devices do now install fully without the tiresome 'try that again' error - so the HERE suite are now handled properly, for example.
So by all means do that backup first, but I was very pleasantly surprised that everything's working fine without any resetting this time around.
It should also be emphasised that the speed of grabbing this update and the Store may well be limited by the number of servers Microsoft has got allocated to Insiders. The bottlenecks I'm seeing look more server-side - obviously the vast Microsoft server farms will be made available when Windows 10 starts rolling out for all form factors through the rest of the year.
In addition to the Store losing its beta flag (and acquiring a new animated banner/carousel of suggested applications, there's the Xbox Music rebranding to 'Groove Music', plus 'Microsoft Edge' as the browser, of course.
PPS. While you're waiting and downloading, make sure you've read my full assessment of Windows 10 Mobile as at June 2015, plus my recent benchmarking features.