Download a scenario you want to edit.

Go to your local scenarios and look up what ♥♥♥ profiles, character profile and map the scenario uses.
You may want to change some values here, too.

Go to the "Customize" section.
Make changes to the ♥♥♥, character, weapon or map you previously have seen being used by the scenario. Rename profiles you edited and save them.

Come back to the scenario's general settings and change its map/♥♥♥ profile/character profile to a new one.

Rename the scenario.
Upload it. Usually takes less than 5 minutes for scenario to get into the list of online scenarios (it will show up after refreshing the list).