# Figure out which commit you want to edit by getting its SHA.
git log

# Start an interactive rebase ($SHA = your commit's SHA and the ^ is important!).
git rebase --interactive $SHA^

# [Change 'pick' to 'edit' for your commit and save the buffer]

# [Add your changes with git add -p, etc.]

# Change the commit and optionally add --no-edit if you want to keep the existing message.
git commit --amend

# Finalize and apply the rebase.
git rebase --continue

# Or cancel the rebase and go back to what it was like before you started rebasing.
git rebase --abort

From Nick Janetakis – Change a Git Commit in the Past with Amend and Rebase Interactive (https://nickjanetakis.com/blog/change-a-git-commit-in-the-past-with-amend-and-rebase-interactive)