A Win7 machine is the easiest way to update the bootloader.
If you have an old bootloader on the K20 of the board, and using Win10 (or Win8), then it will always trigger it to get into bootloader mode.
So you have to update the bootloader with the links I have provided with a bootloader which is Win10 aware. After that you have to load the debug firmware of your choice (J-Link, P&E or CMSIS-DAP).
Unfortunately I don't have your board, so won't be able to check it myself what exactly is on the board.
I hope this helps,
Erich