Yes you can still upgrade for free, as others have mentioned. The easiest way to do this is with Microsoft's Media Creation Tool. You can upgrade to Windows 10 and keep all your apps and files and Windows 10 will be automatically activated. Alternatively, you can do a clean install and activate it with your Windows 8.1 product key. If your PC has an embedded product key, then the Windows 10 installer should read this and you won't need to enter a key manually.