you are currently running a licensed copy of Windows 7 or 8.

1. Go to the Download Windows 10 website. 

2. Under Create Windows 10 installation media, click Download tool now and Run. 

3. 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.)

4. Follow the prompts. 

5. When the upgrade is complete, go to Settings Update & Security > Activation, and you should see a digital license for Windows 10. 

This is taken from the cnet article https://www.cnet.com/how-to/windows-10-dont-wait-on-free-upgrade-because-windows-7-officially-done/

As of the date this post was posted the process works, even though Microsoft said it would end in 2016.