Answer:
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Group Policy, change the setting "Turn off the upgrade to the latest version of Windows through Windows Update," which can be found in Computer Configuration / Administrative Templates / Windows Components / Windows Update.
Doing so sets the registry value described in the rest of this step. Note that for Home versions of Windows 7 and 8.1, where the Group Policy editor is not available, editing the registry is the only option.
To make this change with Regedit, navigate to the following key.
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate
Important: If that key doesn't exist, you'll need to create it.
Create a DWORD value called DisableOSUpgrade and set it to 1. (If that sentence makes no sense, you probably shouldn't be using Regedit.)
