You will need to use filter-branch
git filter-branch --commit-filter '
        if [ "$GIT_COMMITTER_NAME" = "<Old Name>" ];
        then
                GIT_COMMITTER_NAME="<New Name>";
                GIT_AUTHOR_NAME="<New Name>";
                GIT_COMMITTER_EMAIL="<New Email>";
                GIT_AUTHOR_EMAIL="<New Email>";
                git commit-tree "$@";
        else
                git commit-tree "$@";
        fi' HEAD `
If you are the ONLY user who committed to this repository you can simply update all references without checking the old content
git filter-branch -f --env-filter '
    GIT_AUTHOR_NAME="Newname"
    GIT_AUTHOR_EMAIL="newemail"
    GIT_COMMITTER_NAME="Newname"
    GIT_COMMITTER_EMAIL="newemail"
  ' HEAD