If you install Windows 10 after EOS, the Windows 10 Setup will make your system at least unbootable in the best case (if you install Windows 10 to another partition/disk) or completely delete it (if you remove every existing partition).
So if you need to go the route to Windows 10, you first need to make a backup of all relevant data before proceeding.
Instructions on how to create a USB from the Windows 10 ISO: