Apple informed that you must need supported drivers installed with Boot Camp to use Windows 10.And these drivers will automatically downloaded when you use Boot Camp Assistant.Especially you need an ISO file of the 64-bit version of the Windows 10 installer to install Windows on your Mac.
And the good news is Boot Camp also supports upgrading existing 64-bit installations of Windows 7, or 8.1 to Windows 10.For that You need at least 10 GB of free space on your Windows partition. If you have Windows 8.0, update to Windows 8.1 before upgrading to Windows 10.If you have an existing 32-bit installation of Windows on your Mac, you’ll need to back up your important files so that you can remove the existing Windows partition and install a 64 bit version of Windows.
You can install Windows 10 on an internal drive in your Mac. Boot Camp Assistant helps you create a partition on your drive that’s compatible with Windows. When you install Windows on a Mac that includes a Fusion Drive, the Windows partition is created on the mechanical hard drive part of the Fusion Drive (not the SSD).After you install Windows, you can install the latest version of iTunes for Windows to listen to your music or play videos. You’ll need to authorize Windows to play music or videos you’ve purchased from the iTunes Store.