Windows 10's free upgrade offer was supposed to have ended back in 2016. Over three years on, there's still an easy way to obtain a genuine license without paying a cent.
This technique works on the assumption you're currently using a genuine Windows 7 or Windows 8.1 PC. If that's the case, then you can upgrade to Windows 10 by downloading a Microsoft tool and running it on your device. Make sure to backup all your existing files before proceeding with this guide.
Begin by downloading the Windows 10 Media Creation Tool from Microsoft's "Download Windows 10" webpage. This gives you everything you need to perform the upgrade. Run the program once it has downloaded.
After a few moments, you'll see a "What do you want to do?" screen. Select "Upgrade this PC now" and press "Next" to continue with the upgrade. Windows 10 will now install on your PC - this will require several reboots and may take a couple of hours to complete.
Eventually, your device will greet you with the Windows 10 desktop. You're now using a genuine installation of Windows 10, which should remain activated perpetually. You can confirm that Windows is activated by clicking Start > Settings > Update & Security > Activation.
You don't even need to have an existing Windows 7 or 8.1 device to use this trick. The Windows 10 installer still accepts Windows 7 and 8.1 product keys, which are widely available from retailers for lower prices than Windows 10. This means you can even acquire a brand-new Windows 10 license without having to pay the full upfront fee.
As for why this still works, we can't say with certainty. These same steps have been tested, tried and verified by several other news publishers, all with the same positive results. We suspect that usage of this technique is low enough that Microsoft is happy to keep it open and "under the radar," preferring to allow people to upgrade insecure Windows 7 systems and bolster its Windows 10 active users metric.