Determine where in the original presentation you want to create a link to the alternate ending. If you are unsure, you can add a link to a few different slides in the original PowerPoint slideshow. Insert a graphic on the slide that coordinates with the slideshow, or select an existing graphic on the slide.

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Right click the graphic and select Action Settings. The Action Settings dialog box will open. Select Hyperlink To. In the drop down menu, select Other PowerPoint Presentation. Navigate to the alternate ending slideshow in the Hyperlink to Other PowerPoint Presentation dialog box. Select it and click OK.

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In the Hyperlink to Slide dialog, select the first slide of the alternate ending presentation and click OK. Click OK on the Action Settings box.

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