How do I change a commit message?

The most recent commit: type the new message into the commit message field, then press the amend button beside it. What opens is a preview with the old message and the new one side by side, and the commit is not rewritten until you confirm it. Stage nothing first and only the message changes.

The field is not filled in with the old message for you, so this is a retype rather than an edit. Leaving it empty means something different: the existing message is kept, and only the staged changes — or the signing setting — are amended in.

Anything older: Innesta has no editor for a message further back. Changing one means rewriting every commit after it, because a commit id is a hash of its message along with everything else — so the commit changes, and so does every commit that has it as an ancestor.

That is an interactive rebase, which is a command line job:

git rebase -i <commit-before-the-one-you-want>

Innesta notices the repository changing underneath it and reloads, so the rewritten history is there when you come back to the window.