I'll be giving all this a try today because I can't update/upgrade 10TP(posted on another thread @ Ten forums that it is not possible with surfsticks.
Found out at betanews someone posted an ISO at Mega so I'll try that(using my test machine of course and malwarebytes).
So the iso download for build 10041 from Mega is finished, and the scan with Malwarebytes came up clean as did a custom scan to Downloads Library. I'm in the process of extracting to the USB now.
As several people have already said, this only works if the OS you are upgrading is bootable. If not you have to create a bootable install media, DVD or USB flash.
I upgraded my 8.1 Laptop to the initial Win 10 TP by mounting the ISO and running setup from there.
I did it the way David said to, and it booted and installed 10TP in UEFI mode for a clean install, KUDOS David
The setup.exe overrides it all.
You have yet to respond to any of the posts stating this only works if the OS being upgraded is bootable.