You can, at any time, edit a story you wrote.
For this you have to go on the story you want to edit and click the "..." button (1) at the top right of your story.
A menu will appear, click "Edit" (2).
The editing window of your story will appear. You can then edit the text, headings, permissions, and all information about your story.
Once changed, confirm your changes and save. Your story will be published with the changes you've made.
Warning, you can only change the stories you wrote, not what someone else has written.