Probably obvious, but, I did more or less verify that you need a Windows machine to do the firmware update. I've been on Linux and have been Windows free (except for work) for a while now so I tried from Linux first before caving in and digging out an old laptop and installing Windows 10 just to perform the update. I tried both my main Alpine desktop and a Fedora laptop with similar results - it detects the usb connection to the speaker and identifies it as a mass storage device but when I try to do anything with /dev/sdb (in my case), like dd or cfdisk etc. the commands fail with "no medium found".
Neumi tech support responded very quickly and said "The software used to upgrade the firmware only has a Windows version." The one thing I found that makes me think Linux might have or might still work for this was from:
http://www.lemaker.org/product-guitar-guide.html
Which uses the same update tool, albeit an older version, and there is a Linux tool available there, which I tried but it just seg faults. So without an updated tool, or finding an older Linux live cd or something, it looks like Windows is the only option.