Did you make a mistake or is the story not relevant anymore? If you need to unpublish one of your stories, just follow these steps:
Step 1. Click on Content at the top and then go to Stories
Step 2. Find the story you want to unpublish in the list and click on the three dots ... menu
Step 3. Choose Unpublish and confirm
The story will be unpublished from all the channels where you published it, with a couple of exceptions, as for example the Ambassadors channel if the email has already been sent, since we cannot undo that action anymore.