How to change ownership in Windows 10?

I recently purchased another PC (lenovo 535). The original OS was windows 8. The owner had upgraded it to windows10. I would like to replace the title to my own. Is it possible to reinstall windows 10 without having to purchase it.? And if so, would I be able to change the ownership at that time?
At the time of purchase, I changed the password & then decided to go with the pin #. I now have a pin PW, but forgot to write down the text PW & forgot it. I decided to forget passwords altogether but cannot do it without the original PW that I changed. Any solution??

Forget that last post. I decided to reinstall & see what happens.....................................