Editing the git commit message in GitHub

No, this is not directly possible. The hash for every Git commit is also calculated based on the commit message. When you change the commit message, you change the commit hash. If you want to push that commit, you have to force that push (git push -f). But if already someone pulled your old commit and started a work based on that commit, they would have to rebase their work onto your new commit.

GitHub's instructions for doing this:

1. On the command line, navigate to the repository that contains the commit you want to amend.
2. Type git commit --amend and press Enter.
3. In your text editor, edit the commit message and save the commit.
4. Use the git push --force origin example-branch command to force push over the old commit.

Source: https://help.github.com/articles/changing-a-commit-message/