Microsoft is now pushing Windows 10 via a notice suggesting you reserve a copy. Just a week back , he software giant announced that windows 10 will officially roll out on july 29. That means it will be available on any PCs you purchase as of that date and as a free upgrade to your existing win7 or win8.1 pc or tablet. Microsoft has a lot riding on windows 10. The new OS has the task of erasing the bad experiences people had with the touchscreen-focused Windows 8, which ultimately failed to catch fire among PC users. Windows 10 comes packed with new features, such as a redesigned Start menu, the Cortana voice assistant and a new browser called Edge, all designed to win over jaded Windows users. The new OS will also offer a more unified experience among PCs, tablets and smartphones as a way to convince consumers to go the Windows route for all their devices. So how can you now reserve your free copy of Windows 10?
1. First, you must be running Windows 7 Service Pack 1 or Windows 8.1, and you must have installed a windows update dubbed KB3035583. if you have automated updates enabled , the update should already be on your pc.
2. Peek at your Windows system tray in the lower right corner, and you should see a new icon displaying the Windows logo. Hover over it, and the popup messages says: "Get Windows 10."
3. Click that icon, and up pops a window that explains how the free upgrade works. Once it's available, Windows 10 will automatically download onto your PC. You'll receive a notification after the download is complete so that you can choose an appropriate time to install it.
4. When you're done, simply click the button to reserve your free upgrade.
5. The reservation screen asks for your email address so you can receive the notification. Enter your email address and click the Send confirmation button. You can now close the Get Windows 10 window. that's about it !
Windows 10 is a free upgrade. The upgrade offers the full version of Windows 10, not simply a trial or limited version. It also ensures that you can run Windows 10 on your specific device for free "for the supported lifetime of your device," according to Microsoft.

Source : http://www.cnet.com/how-to/how-to-reserve-your-free-copy-of-windows-10/
https://www.microsoft.com/en-us/windows/windows-10-upgrade