To edit a story for your comic book, go to
Story/Feature panel into the New Comic window or into the Edit Comic window.
Click on the appropriate previous or next story button:

to navigate between the previous or next story.
Insert the values for the interested story.
Every time you navigate between stories or add a new story the previous edited story will be saved.
You can notice, high in the panel, the write "1 of 3" or similar that indicates you're edited a story for the comic book.
Save the story as you can save the entire edited comic book.