This is now available via Windows Update.

Just a warning on this update.

As previouslty posted I've download the MSDN ISO when first available and updated on my laptop with no problems at all.

I tried once on my desktop (1x PCIE SSD boot, 2x SATA HDD Storage Spaces, 1x SATA SSD) and it failed with a cryptic error message. Submitted a Feedback to Microsoft and no reply on that. Last night I decided to try again from the ISO and it failed, again. So I opened up the desktop, unplugged the 2x SATA HDD and 1x SATA SSD. The upgrade worked fine. After a subsequent Windows Update (to install this month's normal security updates), I plugged in the 2x SATA HDD and the SATA SSD. All working fine.

Warning: the failed upgrade because of USB storage devices seems to also affect SATA storage devices. 

Microsoft has not replied to my Feedback yet. I can see this will cause a lot of frustration.