Ok, this is a very simple fix, without registry manipulation and running some bogus vbs, cmd or bat, just plain old update pack from updates mirror for windows.
1. You need to to download the following Windows Update Standalone Installer from this instructable or from the original site from following link:
2. Run it, at the end it will ask for a reboot (restart if you are allergic to linux terms xD )
3. Now you got the icon see the image, click it.
4. Once in the Get Windows 10 app, click “Reserve your free upgrade” in the app window.
5. Enter your email if you want confirmation of this reservation. (this is can be skipped).
6. Microsoft will inform you on email and the app will start downloading when it is ready for download, you can wait from some days to some weeks until they will deliver it.
7. More info at http://www.microsoft.com/en-us/windows/windows-10-upgrade
HOT UPDATE (04.08.2015)
• If you, like me, cant wait to have Windows 10 in your hands, you can have it right now. You just need to fresh install Windows 8 on your your PC, be sure to be connected to internet, and after you have installed there will be a offer to upgrade to Windows 10, guess what, Windows Update will download it and upgrade it to Windows 10 right on the spot, no waiting required.