Legally, you can't. Your old license and product key has been "absorbed" by Win10 now. And after 30 days, Win10 deletes the old Win folder, so it can't be rolled back. You can try a clean install of Win8.1 and see what happens, but I suspect it will direct you to buy a new license.
May I ask why you'd want to go back to Win8.1 over Win10? I can see going back to Win7, I did that myself. But Win 8.1?