I don't have a 500 and only recently purchased a 505 but I've had the same "Updating Firmware" screen that wouldn't go away. I recalled reading at some point that re-setting the reader 10 plus times would cause it to revert back to the original firmware (or previous firmware... one of those).
In my case I used a paper clip in the reset hole on the back of the reader and moved the power slider on the top of the reader to the right. The reader would restart and show the same message. I did the same reset over and over and over and eventually, it went right back to the previous firmware.
You could try doing the equivalent with the 500. Be patient. Sit down and watch a half hour sitcom while you do your re-setting. It will take your mind off the monotony.
Good luck.