Windows 10 official release is July 29th 2015. However, you can reserve your copy now through Windows update:
You can reserve your free upgrade in the
Get Windows 10 app. Once you reserve, Windows 10 will download when available, and you can cancel your reservation at any time.
Click on the
small Windows icon found at the right end of the taskbar, if you don't see it , see this page:
https://www.microsoft.com/en-us/windows/windows-10-faq