Click on the Present icon at the top right of the app window to present your story, or press use Command-Enter or Ctrl-Enter on Windows, while in Stories mode. If you are on Stories editor, you will go straight to the story. If you are on the Overflow editor, you will go to the Overflow player, and then you need to click on the story icon at the bottom right corner to enter the story.
Exit back to the Editor by pressing ESC or the "Exit presentation" button.