Is there any way of online editing the commit message in GitHub.com, after submission?
From the command line, one can do
git commit --amend -m "New commit message"
as correctly suggested in the following question:
Trying git pull and then git push has worked (without any other commit having interfered in the mean time).
But can it be done via the GitHub website?
No, this is not directly possible. The hash for every Git commit is also calculated based on the commit message. When you change the commit message, you change the commit hash. If you want to push that commit, you have to force that push (git push -f). But if already someone pulled your old commit and started a work based on that commit, they would have to rebase their work onto your new commit.