Upgrading Windows 8.1 to 10 on a late 2015 iMac via bootcamp
I'm in the process of trying to wrangle Windows 10 onto my iMac. The process that I've seen suggested is to install Windows 8.1 with bootcamp then upgrade it to Windows 10 to avoid problems associated with directly installing Windows 10.
I've successfully installed an unactivated copy of Windows 8.1, but I'm stumped as to how I upgrade to Windows 10. When I try the official Windows 10 installation tool, the installer asks for a Windows 10 product key because "Windows isn't activated on this PC". I don't have any product keys.
Any help y'all can provide would be greatly appreciated.
Model: iMac Retina 5K, 27-in, late 2015.
OS: OS X El Capitan, Version 10.11.6
Processor: 3.2 GHz intel core i5
Memory: 8 GB
Storage: 1 TB Sata disk total, 152 GB of which is currently partitioned into Bootcamp.
iMac Line (2012 and Later)