A week on from build 10549, Windows 10 Mobile is being upgraded again for the intrepid souls in the Fast Ring - the build 10572. This is another forced upgrade from 8.1 though, so yet again there's some juggling of data and lots of spinning cogs before you're back up and running with a smooth new OS. Still, it's what you're here for, eh?

From the Microsoft blog post announcing the new build:

We’re rolling out Windows 10 Mobile Insider Preview Build 10572 today to Windows Insiders on the Fast ring. As you know, build 10549.4 had a bug that prevented upgrade to it from prior builds, so Insiders who wanted to try it out needed to flash back to 8.1. We have fixed this issue in our latest internal builds, and new builds greater than 10575 will have the fix.

So why 10572 if it doesn’t have the fix to allow upgrades? This build has a bunch of great fixes and some very cool new feature additions, and we wanted to give Insiders who really wanted to try it out the opportunity to do so a bit early. Rest assured, if you’re on 10549.4 or an earlier build you’ll be able to upgrade to 10575 (or newer) once it’s out. So if you want to wait for it, we completely understand and it won’t be too long...

Here’s what’s new in this build:

• Get missed call notifications and send texts on your PC with Cortana: With this build on your phone, Cortana will now notify you on your PC (needs to be running Build 10565 or higher) that you missed a call. When you miss a call, you can reply with a text to the person who called you right from your PC and Cortana will have your phone send it.
• If you want to send a text from your PC at any time (even when you haven’t missed a call), just type or speak “Text” and the name of a contact to Cortana on your PC just like you would on your phone and Cortana will take your message and send it from your phone too! Give it a try and let us know what you think.
• For this to work – all you need to do is make sure you are signed into Cortana with the same Microsoft Account on both your phone and PC and Cortana will take care of the rest. If you have multiple PCs and don’t want to receive missed call notifications on one of those PCs, you can go to Cortana’s Notebook and then to Settings to toggle “Missed call notifications” to off on any PC on which you don’t want to receive these notifications. If you would prefer that your phone not send the notification at all, you can go to Cortana’s Notebook and then to Settings to toggle “Missed call notifications” to off and the phone won’t send the notification to any of your PCs.
• Skype messaging, calling and video experience: As we mentioned previously, this build has the Skype integration through the Messaging, Phone and Skype video universal Windows apps. Additionally, the Messaging app now supports animated GIFs and includes the ability to search your messages. And the Phone app has the ability to search for contacts from Call History.
• Cortana improvements: Cortana can help keep track of your leisure time, including movie and event reminders and will give you helpful information to know where to go and to get there on time, including an option to book and track an Uber ride directly from Cortana. With this improvement, you can book an Uber any time you want by saying ‘Book an Uber’ and Cortana will get you a ride immediately. You can even take an Uber to the airport, simply say ‘Book an Uber to SeaTac airport,’ or any other airport recognized by Bing that allows Uber, and have Cortana book you a ride to Seattle-Tacoma airport, JFK, ATL or wherever your travels take you.
• Offline maps: You can now save offline maps to a SD card! This has been a top feature request by Windows Insiders and we’re happy to get it out to you in this build.  
• Photo app improvements: With the most recent update to the Photos app, you can now mark local photos on your phone as favorites and have them shuffle on the Live Tile. We have also made some viewer improvements for zooming performance and file information. And added new launch points for rich editing. We have also added additional context menus (press and hold on a photo). 
• Storage settings improvements: With this build, you will see UX improvements to storage settings on your device that create a common experience across your PC and phone. We have enabled the number one user feedback item for the storage settings page to allow you to install applications to expanded storage such as and SD card or USB drive on BOTH your phone and PC. You can also check out these updated storage settings in Build 10565 for PC we released last week.

The big, huge bug that was fixed, was that Cortana no longer devours battery as it had been doing. Phew! 

The good news is that if you upgraded last week, and if you took my advice and did a clean install, i.e. a hard reset after the upgrade, at least you now have a clean backup that you can restore after this update. (Do make sure that a backup has happened, look in 'Settings/Updates and Backup/Backup/More options'!)

So, in summary:

1. Back up
2. Revert to 8.1 with the Recovery Tool
3. Install Windows Insiders
4. Upgrade to W10M Build 10572
5. Factory reset and choose to restore from your backup from earlier
6. Wait while core apps, Store and Restore have all sorted themselves out (often an hour or so)

So there we go. It's good for the soul. Apparently. Plus it helps Microsoft test the upgrade process that normobs will have to go through from 8.1, so this is you and I being public spirited.

If you're doing all this from a build prior to a fully working 10549 then forget the back up and set up from scratch, re-sync data etc. The cleaner the system the better.

[Update. Well, that was the theory. In practice, something seemed to hose my Microsoft backup and I had to set up from scratch yet again. Sigh. Data points welcome!]

PS. While you're waiting and downloading, make sure you've also read my 'Windows 10 Mobile a step-change, but ultimately needed on many levels', plus my recent benchmarking features.