Support for Windows 7 is now officially over, which means Microsoft wants holdouts to upgrade to Windows 10 to keep devices running securely and smoothly. If you have an older PC or laptop still running Windows 7, you can purchase the Windows 10 Home operating system on Microsoft's website for $225. But you don't necessarily have to shell out the cash: A free upgrade offer from Microsoft that technically ended in 2016 still works.
Here's how to get Windows 10 for free, if you're currently running a licensed and activated copy of Windows 7, Windows 8 or Windows 8.1 Home or Pro:
Go to the Download Windows 10 website.
Under Create Windows 10 installation media, click Download tool now and Run.
Choose Upgrade this PC now, assuming this is the only PC you're upgrading. (If you're upgrading a different machine, choose Create installation media for another PC, and save the installation files.)
Follow the prompts.
When the upgrade is complete, go to Settings Update & Security > Activation, and you should see a digital license for Windows 10."
Above instructions/details taken from: CNET Article
Mod: As pointed out by bigjezza, a digital licence will not be deemed a legal licence, if using you are using this installer to upgrade without a valid windows 10 licence (from a purchase or previous upgrade). From Microsoft: "If you don't have a license to install Windows 10 and have not yet previously upgraded to it, you can purchase a copy here: https://www.microsoft.com/en-us/windows/get-windows-10." For those who do not meet those requirements, install at your own risk.