If you just Executed your Commit and after you find some new Changes to Add, you can Easy Fixing or Amending the Latest Commit with:
git commit --amend
To Execute a Fast Fixing leaving Untouched the Message use:
git commit --amend -C HEAD
This will Add New Changes to the Latest Commit and leave Unchanged the Commit Message.
Indeed the Flag -C serve right to leave untouched the Commit Message when Fixing.
But if you Prefer Instead Removing the Latest Commit for after Executing a New one:
git reset --hard HEAD^
When you need to Edit a Commit Other than the Latest one you can use the Interactive Rebase Command.
In this Example we wanto to Edit the Second Commit before HEAD (HEAD=Latest Commit):
git rebase -i HEAD~2
This will launch the Message’s Editor, so you Edit Message and Save.
After you make the Changes you want to your Commit, then:
git rebase --continue
Try to Fix with a Fetch and Rebase like that:
git commit -a
git rebase origin