Copy/paste into a command line:
git filter-branch --commit-filter '
GIT_COMMITTER_NAME="Your Name";
GIT_AUTHOR_NAME="Your Name";
GIT_COMMITTER_EMAIL="your.email@example.com";
GIT_AUTHOR_EMAIL="your.email@example.com";
git commit-tree "$@";
' HEAD
Note: for more info and fine grane control check out Github's article: https://help.github.com/en/github/using-git/changing-author-info