How to edit incorrect commit message in Mercurial?

Good news: hg 2.2 just added git like --amend option.

and in tortoiseHg, you can use "Amend current revision" by select black arrow on the right of commit button

Well, I used to do this way:

Imagine, you have 500 commits, and your erroneous commit message is in r.498.

hg qimport -r 498:tip
hg qpop -a
joe .hg/patches/498.diff
(change the comment, after the mercurial header)
hg qpush -a
hg qdelete -r qbase:qtip

Update: Mercurial has added --amend which should be the preferred option now.

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You can rollback the last commit (but only the last one) with hg rollback and then reapply it.

Important: this permanently removes the latest commit (or pull). So if you've done a hg update that commit is no longer in your working directory then it's gone forever. So make a copy first.

Other than that, you cannot change the repository's history (including commit messages), because everything in there is check-summed. The only thing you could do is prune the history after a given changeset, and then recreate it accordingly.

None of this will work if you have already published your changes (unless you can get hold of all copies), and you also cannot "rewrite history" that include GPG-signed commits (by other people).