FIrmware refuses to do Updates

> I have a r6400. The underside does not mention V2 [...]

   Sounds like a plain-old R6400[v1].  What's the current firmware
version?

> I am unable to update any firmware [...]

   "unable" is not a useful problem description.  It does not say what
you did.  It does not say what happened when you did it.  As usual,
showing actual actions (commands) with their actual results (error
messages, LED indicators, ...) can be more helpful than vague
descriptions or interpretations.

> [...] Anything I try is refused with the "This firmware file is
> incorrect! [...]

   With my weak psychic powers, I can't see any of that "Anything".
What, exactly, did you do with which file, exactly?