Change commit messages of past Git commits

To change a commit message of the most recent (unpushed) commit, you can simply use
git commit --amend -m 'new message'

To change messages of (unpushed) commits further in the past:

git rebase -i [COMMIT BEFORE THE FIRST YOU WANT TO EDIT]

Mark all messages to be changed with "edit".

Git will start the rebasing and stop at every marked commit. For each of those, do a
git commit --amend -m 'new message'
git rebase --continue