HowTo: Reinstall a new Windows 10 build after reverting to older build

At http://www.askvg.com/fix-windows-10-insider-preview-build-10240-not-appearing-on-windows-update/ it writes:

NOTE:

If you previously had a new Windows 10 build installed in your computer and then reverted back to an older build, you’ll lose access to the new build and it’ll no longer be offered as an upgrade option. Deleting the number for that build from the list at HKEY_LOCAL_MACHINE\SOFTWARE\
  Microsoft\WindowsSelfHost\Applicability\RecoveredFrom key in Registry Editor seems to restore the ability to upgrade. Thanks to our reader "thedicemaster" for this info…

This is useful to know and unfortunately it means that some users may get stuck to an older Windows 10 version. Unless Microsoft has it set like that so that they’ll get automatically the next Windows build (ship the one they were trying to install hoping it fixed the issue they were having). Of course this will work if Microsoft keeps on pushing new builds often, not if they end up updating the OS build once a year or more rare in the future as Windows 10 matures.

To open Registry Editor you can click the Search icon (next to the Windows Start menu icon) at the Windows 10 Taskbar and write RegEdit.exe, then it should offer you a result that says “Run command” that you can click to launch it.

After that, expand the respective tree nodes (that is HKEY_LOCAL_MACHINE, then SOFTWARE, then Microsoft, then Windows, then WindowsSelfHost) from the side pane and right click and delete the “RecoveredFrom” node.

Then try Windows Update again from State menu / Settings / Update & Security and tell it again to check for updates. In case it still doesn’t offer the updates, wait for 1-2 days and try again.