You may have been put off by Windows 8 and may have even downgraded back to Windows 7. You may not be an early adopter and have been waiting for Windows 10 to improve with some updates. If you want to upgrade for free you need to act now.
It has been a year since Microsoft released Windows 10. You have until Friday 29th July 2016 (2 days left at the time of writing this article) to upgrade for free. After that you will need to pay $119 (£90) for the Home version and $199 (£150) for the Pro version.
If you are on Windows 7 or 8 and you let it regularly install the Windows updates, you will probably have notice the White Windows icon in the bottom right hand corner next to the clock. It pops up with a message now and again telling you to upgrade to Windows 10. You should be able to click on that and go to the screen in Windows updates to start the install. If you don't see it you could try doing a full windows update to catch up with all of the windows updates you have missed.
Alternatively, you can google "Microsoft Windows 10 download" and that should give you results from microsft.com
Follow the instructions on there to either download and install it on that machine or download it to use and install it onto another PC.
Please share this with your friends and family. They may have forgotten so you could be saving them a decent amount of money.