Enable edit mode, go to the scene*, press CTRL+E and edit it. When oyu're done, press CTRL+E to save, then upload the <scene_name>.sco file in your module's SceneObj folder.
* In modding, map is called the world map in single player, where you walk around with your little horse icon. When you control your character, it's called a mission and the place is called a scene. Please use these terms to avoid confusion.