GIT: How to change git commit message after push?

If it is the most recent commit, you can simply do this:

git commit --amend
This brings up the editor with the last commit message and lets you edit the message. (You can use -m if you want to wipe out the old message and use a new one.)

And then when you push, do this:

git push --force <repository> <branch>
Be careful when using push --force. If anyone else has pushed changes to the same branch, those changes will be destroyed.

Anyone who already pulled will not get an error message, and they will need to update (assuming they aren't making any changes themselves) by doing something like this:

git fetch origin
git reset --hard origin/master # Loses local commits
Be careful when using reset --hard. If you have changes to the branch, those changes will be destroyed.

A note about modifying historyEdit
The destroyed data is really just the old commit message, but --force doesn't know that, and will happily delete other data too. So think of --force as "I want to destroy data, and I know for sure what data is being destroyed." But when the destroyed data is committed, you can often recover old commits from the reflog—the data is actually orphaned instead of destroyed (although orphaned commits are periodically deleted).

If you don't think you're destroying data, then stay away from --force