Thank you all of you. I was able to redo the commit.
But as Tony has pointed out. I would have needed to redo all the subsequent commits if I was to change non-HEAD commit message. What is the proper way of doing that? Is it the same as Documentation/howto/revert-branch-rebase.txt ?
One more question is that, how to use the git commit --reedit-message flag? According to Documentation/howto/rebase-and-edit.txt, I guess the meaning is to re-apply one commit to current HEAD?