From the docs for
Undo a commit and redo
$ git commit ... (1)
$ git reset --soft HEAD~1 (2)
<< edit files as necessary >> (3)
$ git add .... (4)
$ git commit -c ORIG_HEAD (5)
This is what you want to undo
This is most often done when you remembered what you just committed is incomplete, or you misspelled your commit message1, or both. Leaves working tree as it was before "commit".
Make corrections to working tree files.
Stage changes for commit.
Commit the changes, reusing the old commit message.
reset copied the old head to
-c ORIG_HEAD will open an editor, which initially contains the log message from the old commit and allows you to edit it. If you do not need to edit the message, you could use the
-C option instead.
Editor's note 1: You don't need to reset to an earlier commit if "you misspelled your commit message". If you
reset, git will not link new activity to the previous commit in any way, giving you a blank slate for a new commit message. The easier option is
git commit --amend, which will open your default commit message editor pre-populated with the last commit message.
Beware however that if you have added any new changes to the index, using
commit --amend will add them to your previous commit.