Windows 8.1 and Server 2012 R2 Previews
Windows 8 Home users can upgrade here
Should you want to give Microsoft feedback on your testing, you can sign up at Connect, but I don't think they are taking 8.1 testers right now.
You will also be able to download 8.1 from the Windows App store if you already have Windows 8. There will be a notification that will show up for Windows 8 users to upgrade.
Windows 8.1 will be a free upgrade for licensed Windows 8 users.