Amend the previous commit that has not been propogated (i.e. undo it, then start a new one with the pending changes in the index):
git commit --amend -a -m "New commit message"
Undo a commit that has not been propagated:
git reset --hard HEAD^
Once a commit that has been propagated, there is no way to undo. However, the following will create a new commit undoing the previous commit's changes:
git revert HEAD
