To change the commit message of the latest (unpushed, unmerged) commit, you can use
git commit --amend
To change the commit message of an earlier (unpushed, unmerged) commit [COMMIT], you can do
git rebase -i COMMIT~
For a current version of git, you can simply mark the line with "reword", and git will ask you later for the new message.
For older versions:
edit
git commit --amend
when rebasing stops at the relevant commitgit rebase --continue