So today was the release date of Windows 8.1 RTM, which is the version released to the manufactures. Device manufactures which are included when selling their devices.
In other words, it is as close to the version we can buy as it gets.
Windows 8 Pro wil get the Upgrade as an Update (like Service Pack) trough Windows Store. Access from your modern UI if you logon with a Microsoft ID. Its free.
Windows 8 Preview wil not get an upgrade, but trough MSDN or similar we can download the ISO image. Personal files, settings and apps from Windows Store will be copied.
Windows 8 Enterprise doesn’t have an upgrade, but if install Windows 8.1 Enterprise from the earlier mentioned ISO images it will keep all personal files, settings, apps and third party installed software.