I tried that, but it didn't work. Maybe because I saved the game under a new name in the meantime?
(I always save files followed by a prefix number counting up, like "Game01", "Game02", "Game03", and so on, to keep track of my progress as well as to be able to go back to a previous version if something goes terribly wrong. Before publishing I give it the proper name, but delete that version right afterwards to not mix up my prefix numbers.)
If that is what's causing the problem, then I have a suggestion: The option to replace an existing game should appear when the one you're publishing has the same name.