Edit the last thing you committed to git
I use this all the time, it's perfect for when you commit, then realise you need to change one small part of your code.
Make your changes and add them, next run commit with the --amend flag and it will update your last commit with the changes you just added!
git add --patch
git commit --amendWarning: Never amend commits that have been pushed to a shared branch as it can cause issues if someone has already pulled the original commit.