Sometimes, you just don't get it right the first time, and you need to undo your last commit. Perhaps you accidentally added files you didn't want to, or just want a clean slate.
Luckily, your problem is easy to fix. Simply run this command:
git reset --soft HEAD~
This will reset your repository to the moment before you committed. If you want to remove all the files that were staged as well, you can:
git reset HEAD~
Simply make the changes you needed, add your files, and commit again.
If you want to change an existing commit rather than undo it, you can use git --amend: How to Edit the last git Comment