I finally figured out my problem and it was totally a rookie mistake. I kept renaming the file atari_update.img which is what all of the instructions I read said to do. However, the file extension (.img) is really already set up, so when I renamed the file I really only needed to type atari_update. When I could not get it to work it was because I was effectively titling the file atari_update.img.img. Once I corrected that on my SD Card everything worked exactly the way it should have.