Because it's not very obvious, I keep forgetting how to edit the story's main text and photo. Would someone please help me out? Never mind, I've figured it out.
Just for the reference: Option 1: Go to "My Stories", find your story, click on the ellipsis beside it, and choose "Edit Story". Option 2: Go to the story map of your story. In the address bar of your browser, replace the "map" with "edit" and press "enter."