Titles of slides are shown in the Outline tab of the player. By default, titles are taken from the PowerPoint slide titles of the presentation. To change a slide title, click on it or press F2, and type a new one. The new titles will be displayed in your presentation navigation. They can differ from PowerPoint slide titles. Note that if you then replace the title on the slide itself, then the published presentation will still display the one you specified in the Slide Properties window.