The previous Insiders 'fast ring' build was something of a disaster, not least because you had to jump through a few hoops to get it to install, but we have a new Redstone 2 build today anyway. It's not clear whether you might have more luck with this or whether the same hoops are required (if not already done) - data points welcome!

From the Microsoft blog post:

Today we are excited to be releasing Windows 10 Insider Preview Build 14955 for Mobile and PC to Windows Insiders in the Fast ring.

What’s new in Build 14955

Outlook Mail & Calendar Update (PC & Mobile): We have updated the Outlook Mail and Calendar apps to 17.7466.4062x.0.. This update introduces a few new things for you to check out:

• You can now open email messages in a new window.
• You can now use Quick Actions directly from new email notifications.
• Get someone’s attention with @mentions. Type the @ symbol anywhere in the body of your email and start typing to pick the person you want to address.

Other improvements and fixes for Mobile

• We fixed the issue causing you to be unable to sign into apps such as Feedback Hub, Groove, MSN News, etc. with your Microsoft Account if you sign out or get signed out of these apps.
• We fixed the issue that was causing some devices to appear “stuck” on the blue Windows logo screen during the boot cycle when rebooting a phone on earlier builds.
• We fixed a bug resulting in notifications banners not appearing from Outlook Mail accounts that had been pinned as tiles to Start.
• We fixed an issue resulting certain apps being unexpectedly missing from the Share dialog.
• We fixed an issue where Insiders who had configured a data limit on their phone may have ended up in a state where they would receive frequent notifications about having exceeded that limit.
• We fixed an issue where the horizontal progress dots (as displayed when using the indeterminate ProgressBar control, for those who are familiar with XAML) were incorrectly fading out at the right side.
• We fixed an issue resulting in not being able to enter input into the Time or Place fields when creating a Reminder in Cortana.

Known issues for Mobile

• If your device never successfully updated to Build 14951, please see this forum post. If your device did receive Build 14951, you should be able to download and update to this build just fine. THANK YOU Insiders who helped us with getting the 0% download issue fixed last week!
• Some Insiders are receiving an 0x80242006 error trying to update to the latest builds. We are investigating – see this forum post.
• You will be unable to install additional languages, keyboards, and speech packs on your phone for the next few weeks. If you have existing languages, keyboards, and speech packs installed – they will carry over when you update to new builds. You just can’t install any new ones. If you do a hard reset of your phone on these builds – you will also be unable to install additional languages, keyboards, and speech packs. You can use Windows Device Recovery Tool to go back to Windows Phone 8.1 or Windows 10 Mobile, install any languages, keyboards, and speech packs you need and then update to the latest build in the Fast ring as a workaround.
• Excel Mobile will freeze after adding sheets and eventually crashes.

I went through the few minutes of hoops on my 950 XL and received 14951 in the end, and today's 14955 installed happily over the top. In theory, those who had problems should be OK with this new build, but if all else fails, see the simple steps in the forum post linked above.

Of course, in the fulness of time we'll need a summary of what's new going from Redstone to Redstone 2, but we won't have a full feature set or changelog for a while yet - this mobile OS has got legs, even if the number of licensees isn't overwhelming. Settle down for the long haul, everyone....!

Anyone with an existing phone that's officially deemed Windows 10 Mobile compatible by Microsoft can join in the fun. The full list of phones includes Lumia 430, 435, 532, 535, 540, 635 (1GB RAM version), 640, 640 XL, 730/735, 830, 930, 1520, Icon, BLU Win HD w510u, BLU Win HD LTE x150q, and the MCJ Madosma Q501. Plus, of course, all the devices which have shipped with Windows 10 Mobile in the last year, of which the Lumia 550, 650, 950, 950 XL, Acer Jade Primo and HP Elite X3 are the highest profile.

In each case:

1. Upgrade to Windows 10 Mobile (if appropriate) using the Upgrade Advisor.
2. After at least one update cycle you should then be on the Anniversary Update (a.k.a. Redstone). Head into Settings/Update & Security/Windows Insider Programme
3. Join the Insiders programme and choose the 'Fast' level in the pick list.
4. Within a few hours your phone should be seeing the build mentioned above and you'll be running next year's mobile OS today. Fun, fun, fun....!