git reset --soft HEAD^
git reset HEAD^
//And if you actually want to completely undo it, throwing away all uncommitted changes, resetting everything to the previous commit (as the original question asked):
git reset --hard HEAD^
git reset --soft HEAD^
$ git reset --hard HEAD^
git reset --soft "HEAD^"
git reset --soft "asdf"
$ git reset --soft HEAD^
$ git reset HEAD~2
$ git reset [commit_hash]
$ git reset “HEAD^”
$ git reset “f145h7r”