Windows 10 will be released on July 29th. Windows 7, 8, and 8.1 users are eligible for a free upgrade to Windows 10.
On the bottom right of your screen you should see this icon:.
This is the symbol for the Windows 10 upgrade. Go ahead and click it. Fill out your email address or just click that you want the upgrade without providing your email address. When Windows 10 is released, you will get the upgrade. It might take a few days for the software to download to all the people that have requested it. I have played with a pre-release version and outside of not being able to test Minecraft on it for a client, it worked well. BUT...
MY RECOMMENDATION RIGHT NOW IS TO SIGN UP AND DOWNLOAD BUT DO NOT INSTALL THE UPGRADE. In fact, I would wait until September at the earliest to update. Let others find and battle with the installation bugs. But, if you do feel like being a Microsoft early adopter and not heed my recommendation, then make sure you are backing up your information just in case you have a catastrophic issue.