Have you considered installing the regular Windows version for wich you have the license?
You could do a fresh install of Windows and then install all the drivers and software you wish from Dell's support page.
That's what I did here to convert my steam machine to a regular Windows based desktop PC.
In my case I chose not to install any software or driver from Dell.
If you choose this path you can download Windows 10 usb tool from www.microsoft.com/en-us/software-download/windows10
and follow the instructions to create a usb installation flash drive.
Remember to choose the version of Windows 10 you have the license for and not the one that comes preinstalled on the Alphas (if they are diferent you would not be able to activate it with your key).
If I remember correctly there is even an option to insert your key before choosing the Windows version so it downloads the correct version for your license.
The tool will then download the Windows image you chose and configure the flash drive to be bootable.
Then you just have to boot up you steam machine from the usb disk and follow the instructions.
I suggest deleting all the disk partitions you have during the initial configuration of the installation process.
All necessary Windows 10 partitions will be automatically created if you do it.
Remember this would delete every data you already have in it so a backup may be appropriate before you begin.
Good luck!Edited by cborw - 9/1/16 at 12:52pm