I just finished updating my R6 Mark II. It was frustrating because I normally use the method of formatting and SD card, adding the new firmware to it, and then using the camera itself to find and install the update. This time, I could not get the camera to "see" the new firmware (.FIR file) on the SD card. After numerous iterations of debug, I ended up just using the EOS Utility 3 application on my Mac (MacBook Pro 14", M2 CPU, 16G RAM, OSX Sonoma 14.6). That worked out, finally. However, I did lose 30 minutes flailing around with SD cards before that.
BTW, if you need to use EOS 3 like I did, it is important to know that all the instructions in the pdf file that comes with the update need to be followed pretty literally. This includes any error messages. The Canon software is pretty touchy. I was getting an error message that "USB connection cannot be used because it is in use by another app" or some such. It was Photos which had launched as soon as the camera was connected. I shut Photos off and then was faced with the gallery screen (where EOS Utility displays photos that are on your camera to select for download and such). I had to kill this to see the main initial screen that has "Camera Settings" as a choice. I had to figure out to close the gallery to see it because the app does not list all the open windows on the "Window" menu item. Also, Canon left "View" pretty useless as well. I'm mentioning this because the app may seem like it is completely not working, but in fact it is just has a super marginal UI - which you know if you are a long time Canon person.
I'm up and running now, so this is just a PSA.
