If you have legit Win8.1 with you, then just do an upgrade. Download an official Windows 10 ISO (like, the Insider Preview build 10240), extract its contents on your HDD run the setup. It'll automatically upgrade your Win8.1 to the latest. Since Win10 uses motherboard (UEFI more specifically - since it saves the key in your hardware) to bind the license, it won't be a problem for you if you upgrade. If your lappy doesn't have UEFI, then you might run into trouble if you do a clean install.