You get the usual Git error "unable to fast-forward" which means in plain English that what's on the remote repo doesn't match what you have locally anymore, so you can't push, unless you force.

In such cases, GitUp just prompts you if you want to force push.

If you decide to force push, then your updated commit message is on the remote repo as well. This is completely safe if this is your private repo, or you are pushing to a private branch. In any other situation e.g. a shared repo with a team or a public repo on GitHub, you should pretty much never force-push as this will mess up other people clones (as there are now 2 histories of the repo in a way).