Note: This answer is being posted appropriately 4 years after the above question was asked. Therefore, this answer is more focused on users with a similar question and have more newer versions of macOS available, but desire to continue to use an older version of macOS.
If newer versions of macOS are available, then one option would be to erase the Mac and install the latest version of macOS available for your Mac. Next, download the latest release of Windows 10. Use the Boot Camp Assistant to install Windows 10. Finally, erase your current macOS and install the desired older version of macOS. If erasing your current macOS is not a viable option or this procedure fails to work, then try the next option.
Use the Boot Camp Assistant to install a version of Windows 10 released about the same as your current macOS. Wikipedia has the release dates for macOS and Windows 10. From Windows, download the current Windows 10 release and use the download to upgrade Windows 10.
Rufus can be used to download older Windows 10 releases. You will need Windows 7 or later in order to run Rufus. If a machine with Windows installed is not available to run Rufus, then you could temporarily install the current Windows 10 in a virtual machine. A temporary installation of Windows 10 in a virtual machine does not require a product key. Both Oracle VirtualBox and VMware Fusion are free virtual machine products. If the current virtual machine software is not compatible with your current macOS, then try installing an older version of the virtual machine software. Once you have acquired the desired Windows 10 ISO, you can delete the Windows 10 virtual machine.