It's not a rollback of OS version, but an option to rollback to a 'previous build'. DANG I shut off my Win10 just before coming here, for I just woke up. Will edit this post with the steps after I reboot that machine.
EDIT: first look at this article. It has some fixes.
Then, go to Control Panel, again select Troubleshooting then Windows Update. Be connected to the internet when you do this (were you connected before?). It runs a diagnostic and when I did it, even though I get the updates fine, it said there was an error it fixed (in gibberish, I don't understand what the problem was). Takes maybe five minutes. Then wait a bit, then query for updates, see if your 10240 comes.
If not, then try these steps...
Notification icon, All Settings.
Then, at very lower left, 'Update and Security'.
Then, at left see Recovery, which lets you "Go back to an earlier build."
OR, top left, 'Windows Update.'
Then 'Advanced Options'.
Then, 'View Your Update History'
Then, second in the list, 'uninstall latest preview build'.
Now, I'm hoping that after uninstall, your rolled-back build will result in being prompted with the next build; which, maybe is now 10240, rather than 10166.
The whole menu structure is so off-putting with just a few options each time, forcing you to use page after page in an unintuitive manner. Whoever designed it, should be fired. Yesterday. I understand they're trying to simplify, but it's actually simpler if they had a one-page Table of Contents, linked. Then in group policy, the admin could choose which options won't show up, or at least which should be greyed out.
Edited by brainout, 27 July 2015 - 04:07 AM.