Okay, I just found something which may fix the problem.
Did you unzip the ROM file using Windows built-in unzipper?
If yes, install trial version of WinRAR. Go here and scroll down to the sub-heading halfway down: "English WinRAR and RAR release"
Choose either WinRAR x86 (32 bit) 5.01 or WinRAR x64 (64 bit) 5.01 as appropriate for your Windows OS.
Install it.
Now right-click the BIOS zip file and unzip/extract the contents with WinRAR this time, using the right-click context-menu (first make sure you have deleted the first ROM file so you don't try to use it again).
Now try updating the BIOS using same method as before.