I'm here to clarify the steps taken for the .rfu file in the firmware update readmes.
Download the firmware .zip file offered on the site for all operating systems. Unzip it and put it somewhere you can find it easily like the desktop or documents folder.
Next, open up internet explorer and type in ftp:// (your printer's ip address like 192.168.3.25) and enter it. You'll come to a page that has a very meager amount of words on it and should literally be titled FTP root at (ip address). Here is where you find the EXACT phrasing "To view this FTP site in File Explorer: press Alt, click View, and then click Open FTP Site in File Explorer. " Following the page's instruction should open the ftp root in a File Explorer window from Windows Explorer, which is literally coming from YOUR/YOUR PRINTER'S RAM Disk (idk which, doesn't matter).
If you can't get past this, try putting the ftp address in Windows Explorer instead (it also has a url bar, but is mostly for local searches). Proceed to drag and drop the .rfu file to that new window ino the PORT1 folder. Watch the magic happen.
If the firmware is already up to date or you still don't have proper admistration to update this printer, an error message will display on your computer monitor. Seek IT assistance.