If you’ve unknowingly downloaded and run ‘WindowsTechnicalPreview.exe file’ from this page provided by Microsoft by clicking on ‘Get the upgrade’ button, later, if you’ve realized it was going to upgrade you from current OS to Windows 10 Preview and cancelled the setup, (you can download TP ISO from here and install on a new partition).

But, Windows Update may have already started prompting you to install ‘Update for Windows’, which you don’t want to do it, (hiding it doesn’t help) because it upgrades your Windows 7 or Windows 8 or whatever you’re on to Windows TP.

Avoiding Windows 10 preview from being installed via Windows Update 

1. Control Panel > Programs > Programs and Features > Installed Updates

2. Search for KB2990214 update in the search bar and Uninstall it.

3. Restart your computer and run the Windows Update again.