Maybe you simply want to mix things up. Or perhaps your presentation data lends itself more to a portrait orientation. Either way, you can change the orientation of your slideshow slides from landscape to portrait.
First, ensure you’re in Normal view. Open your slideshow and select the View tab on the main ribbon. In the Presentation Views group, click on Normal. Next, select the Design tab on the main ribbon. In the Customize group, click on Slide Size. Choose Custom Slide Size. In the resulting Slide Size dialog box, click on Portrait in the Slides area. Click OK. In the resulting Microsoft PowerPoint dialog box, choose to either maximize your view, potentially losing some data around the edge, or you can ensure that all data will fit on the slide.
Be aware: This technique changes the orientation of every slide within a slideshow. You can’t switch between portrait- and landscape-oriented slides within the same presentation.