Microsoft is making Windows 10 available as free upgrade for qualified Windows 7, Windows 8.1, and Windows Phone 8.1 devices. It will be available starting July 29, 2015; people can reserve their free upgrade today.
You only have until July 29, 2016 to take advantage of this offer. Once you upgrade, you have Windows 10 for free on that device.
Yes, it’s free. This is a full version of Windows, not a trial or introductory version. It is available for a limited time: you have one year from the time Windows 10 is available to take advantage of this offer.
You can do one of two things*:
a) Buy a new Windows 8.1 device and upgrade it to Windows 10 for free, or
b) Reserve a free upgrade for your existing Windows 7 or Windows 8.1 PC
Not every device will run every feature of Windows 10. Be sure to check with your manufacturer before purchasing. Additional details on upgrade qualifications are below.
When you reserve, you can confirm your device is compatible with Windows 10. Between reservation and when your upgrade is ready, the files you need for the upgrade will be downloaded to your PC to make the final installation go more quickly. Then, when your upgrade is ready after July 29, 2015, you get a notification that lets you get started with your upgrade.
The only requirements are that a) your device is compatible, and b) you’re running genuine Windows 7 Service Pack 1 (SP1) or Windows 8.1 (Update).
Windows 10 is designed to run on Windows 7 and Windows 8.1 PCs. That means your device is likely compatible and will run Windows 10.
Yes, new qualified PCs can run Windows 10. Windows 10 is designed to be compatible with the Windows 8.1 laptop, tablet or desktop PCs that are for sale now. Device manufacturers make decisions about their device models and some Windows 10 features may not be available on certain devices, so be sure to check with the retailer or manufacturer for details.
Plan for about an hour for the installation. Newer devices may only take 20 minutes, and older devices may take more than an hour.
You’ll need to download the installation files before you begin. If you reserve, we’ll take care of downloading those files for you and let you know when you are ready to get started.
you can get more information from Microsoft’s FAQ here