Yep, there is also the CD install option - using the ISO file - you would need to burn a CD on slow - but I'm not going to advise it, as if there is a problem, the CD can get stuck, then it's unit out, disassemble, reverse power the CD to get the thing to eject!
These bloody RT6 systems are total pants - I've never had so many issues with people trying to undertake what should be a simple firmware update.
Taking it to the dealer was going to be my next suggestion. You have the same file sizes, so the system does not like the USB / files, so I would suggest you ask them top undertake the firmware update to 2.86. Also check to see if the bluetooth calibration file has also been applied (to correct the reversing sensor volume being too low / high and the echo heard by recipients whilst on a bluetooth call). I suspect that is might have been, given the firmware version you are on - but I wouldn't leave it to chance - get them to check Rick.
You may need to call ahead as they may need to order in the CD for the firmware - but this should only be a couple of quid - you can then decide whether you want to install it yourself or if they do it, they may charge. The calibration file itself, is downloaded from their servers Rick.