The following bash script will change the author information for all commits in a repo. Becareful using this on public repos!
#!/bin/sh
git filter-branch -f --env-filter "GIT_AUTHOR_NAME='newName'; GIT_AUTHOR_EMAIL='newEmail'; GIT_COMMITER_NAME='newName'; GIT_COMMITTER_EMAIL='newEmail';" HEAD
Once this has run successfully (you can check this by running 'git log' and press wq to exit the log). Run the following git command:
git push -f origin master