Change the git author data (username + email) and then run:
git commit --amend --reset-author
When the editor exits, the new author will be saved to that commit.
When trying to change the author of more than just the most recent commit, git rebase must be used.
The process for this goes something like this.
Step 1: Start rebase
# Replace X with the number of commits to go back
git rebase HEAD~X
Step 2: Select edit for all commits
Step 3: For each commit, reset author then continue rebase
git commit --amend --reset-author
git rebase --continue