When you first create a story in the story list, you provide a name for your story. If you'd like to change it, choose Rename Story from the story menu in the story map.
New stories start using the default format you set in the story list using the Formats menu item, on the right. To change an individual story's format, choose Change Story Format from the story menu in the story map. Tick the checkbox next to the format you'd like to use.
If you'd like to add a new format that isn't listed on this dialog box, you can do so with the Formats menu item in the story list.