Upgrade Windows 8.1 to Windows 10
I updated from Windows 8.1 to Windows 10 on a MacBook Pro 15 inch Retina 2013.

Best is not to do a clean install but choose to upgrade the existing OS, Windows 7 or Windows 8.1. This will avoid a lot of problems with the authentication key you need for Windows 10 and the necessary drivers.

Windows 10 will use the Bootcamp Windows 8. 1 drivers which are already installed on your PC. They all work ok except for the bluetooth driver.
It is installed but bluetooth doesn't show up. That's the only problem I had with upgrading to Windows 10.