Verify if you have the November Update installed:
How to verify if you downloaded or installed the Windows 10 November Update
If you do, then all you need to do is change the product key using your Windows 7 or 8 product key:
Click Start > Settings > Update & security > Activation > Change the product key
If you don't have the November Update, download it then perform a repair upgrade:
How to download official Windows 10 ISO files
then
How to: perform a repair upgrade using the Windows 10 ISO file
also review
How to install and activate Windows 10 using your Windows 7 or Windows 8 product key
also
How to Find Your Product Key After Upgrading to Windows 10