@djlf This is how you fix it. In the command line run cd RetroPie-Setup then
git reset --hard
This will remove the change that was made preventing it from updating. What else it will do to your system I cannot predict or know.
Then, if you are determined to update just update normally.
DISCLAIMER: ANYTHING THAT HAPPENS TO YOUR SYSTEM WHEN RESETTIING GIT OR UPDATING IS TOTALLY ON YOU