I was unable to find a WUSB100-RM on the manufacturers website. However, I found one similar, it's the WUSB100:
I believe the "RM" at the end of the model number indicates it's a R
anufactured product. That would account for the reason why a model with RM is not listed in the drivers download section.
I wasn't aware that this device had upgradable firmware available. I'm certain that it does not. What problem are you having that you believe firmware would fix?
It gives better performance if you use the drivers the manufacturer provides over using the Windows networking software. I'm puzzled about why you would want to attempt that task.