For all Volume licenses (Enterprise) you need to contact the volume license center.
You cannot "upgrade" from win 8/8.1 Enterprise to win 10 enterprise'
If you are running the Enterprise edition of the Windows 10 Insider Preview, to upgrade to the final release of Windows 10 Enterprise you will need to download and activate it from Volume Licensing Service Center. As a reminder, the Enterprise edition of the Windows 10 Insider Preview is not eligible for the free upgrade offer and can only be upgraded to the final release of Windows 10 through an active Software Assurance agreement.
Barney93,
You have it in reverse, Enterprise is basically "the corporate version of windows 10" and you cannot "upgrade Enterprise to 10" it is already part of the Windows 10 version and is basically "at the top of the pile with all the security and other stuff needed for corporations and not necessary for the home user"
Hope that answers your question. BTW it is also the most expensive version and can only be licensed as part of a Software Assurance agreement in the Volume Licensing Service Center