Windows 8 Upgrade is an option to install Windows 8 on a computer that already has an earlier version of Windows installed. It is handled like an update and only the system files that changed from the old system to Windows 8 will be changed. The user data will not be deleted or affected in any way.
The Windows 8 upgrade can be performed only from a computer with a compatible earlier version of Windows (Windows XP, Vista, or 7). An upgrade from XP will preserve only user data, while an upgrade from Vista will also preserve Windows settings, and an upgrade from Windows 7 will preserve installed programs as well as Windows settings and user data.