You can use most product keys from Windows 7 and Windows 8 to activate Windows 10.

Different ways of activating windows 10.

1. right-click start menu, scroll to bottom and click “change product key or upgrade your edition of windows”, Click “Change product key”
2. Control Panel, System, Click “Change Product Key”
3. Run command “SLUI”
4. Run command “changepk”
5. Open elevated command prompt, type “slmgr /ipk xxxxx”, replace the xxxx with your product key. If it was successful,  a window will pop up saying so.