Re: how do I reserve windows 10 for my lap top which has abuntu os?
Reserving a copy of Windows 10 is limited to those who have Windows 7 SP1 or Windows 8.1 installed and wish to upgrade for free. If you would like to install Windows 10 from a different OS, whether it is Ubuntu, Linux Mint, Vista, or XP, then you will need to purchase a copy of Windows 10.
Windows 10 is available for purchase from most major retailers that sell computer software, such as Best Buy, Amazon.com, Office Depot, Staples, and many more. You can get it on either DVD media or a USB flash drive media.
If you have a valid license for Windows 7 or 8.1 with the media, and have just replaced it with Ubuntu, then you would need to re-install that operating system in order to take advantage of the free upgrade, as you need to perform the upgrade from within Windows 7 or 8.1.
See also,
http://forums.windowscentral.com/windows-10-how-guides/371953-how-get-started-windows-10-a.html