git config global file - remove settings
Solution 1:
You can edit the ~/.gitconfig file in your home folder. This is where all --global settings are saved.
Solution 2:
Super late answer, but hopefully still helpful:
git config --global --unset-all user.name
Then you're free to:
git config --global --add user.name <whatever>
Solution 3:
git config --global --unset-all user.name
Or you can just change the user name like this:
git config --global --replace-all user.name "New User Name"