Microsoft announced the Windows 10 OS just 1 day ago. But it is not available for download yet. If you want to get a download link for Windows 10, you can join the Windows Insider Program. Microsoft is giving early access to those who are part of the Insider Program.
The Insider Program is expected to be launched very soon. I am not sure how many people will be allowed to join this program and test the Windows 10. If you are keen on early access to the new OS, you are recommended to join the program as soon as it is launched. Keep an eye on http://preview.windows.com for more details.