To change your PowerPoint slideshow orientation from horizontal to portrait, you must access the "Slide Size" feature under the "Design" tab.

In PowerPoint, slides are sized for an onscreen slide show. They are displayed horizontally, with slide numbers starting from one, but there are features in PowerPoint that allow the user to change ...

You can change the size of a slide in your PowerPoint presentation through the "Design" menu on the Windows PC, Apple MacOS, ...