1.) Click on the windows icon in your toolbar at the bottom right of your screen on your desktop
2.) Once you click on this icon, a window should pop up that looks like this! Click on "Reserve your free upgrade"
3.) Once you click on that button, the window will look like this! Enter your email for confirmation.
4.) You should recieve an email from Microsoft saying you have reserved your free copy of Windows 10! Congratulations! If you click on the little windows icon again in your toolbar, a window like this will pop up.
(Disclaimer: all copyrights are owned by Microsoft, and am just sharing the good word about them)