How to change your commit messages in Git?

Case 1 : Not pushed + most recent commit:

git commit --amend

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Case 2 : Already pushed + most recent commit:

git commit --amend

git push origin master --force

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Case 3 : Not pushed + old commit: 

git rebase -i HEAD~X

# X is the number of commits to go back

# Move to the line of your commit, change pick into edit,

# then change your commit message:

git commit --amend

# Finish the rebase with:

git rebase --continue

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Case 4 : Already pushed + old commit:

Edit your message with the same 3 steps process as Case 3

(rebase -i, commit --amend, rebase --continue).

Then force push the commit:

git push origin master --force

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