Upgrade boot camp from Windows 8.1 to Windows 10
It has been a while since I use windows 8.1, but I believe you can mount the Windows 10 iso. From there you can just click on the setup application and upgrade to Windows 10.

Some comments on what you are attempting.


I understand you dowloadeded a windows utility to create the bootable USB. You make not mention copying the Windows Support Software to the USB. This is required.
You see three options: Mac, Windows and the USB drive. The correct answer is "EFI Boot". I assume this is what you meant by USB drive.
The Boot Camp Assistant creates the partitions necessary for the installation of Windows. You make no mention of creating space for the Windows partitions.
All Microsoft "Media Creation Tool"s are incompatible with Apples way of installing windows. In other words, you should not use a Microsoft "Media Creation Tool" to create the USB Windows installer. You can use a Microsoft "Media Creation Tool" to download the Windows iso file.
Your Mac already contains all the tools needed to create a USB Windows installer from an Windows iso file. You should use the Boot Camp Assistant to download the Windows Support Software, but from there you can create the USB installer without the aid of the Boot Camp Assistant.
Actually, you can install Windows 10 without using any USB drives. A procedure for this is given here.


If you do not have working Windows 10 iso file and you have activated your Windows 8.1 installation, then you can upgrade to Windows 10 for free. This offer expires at the end of the year. You do have to agree that you use "assistive technologies". (Microsoft will not check to see if you do). The web site to see is: Customers who use assistive technologies can upgrade to Windows 10 at no cost.