This article explains how to change all the slides in a Google Slides presentation from landscape (horizontal) mode to portrait (vertical) mode and then back to landscape mode again.

After you use Google Slides to create a presentation, it only takes a few moments to go back and change the orientation of a slide from landscape to portrait. This process changes the orientation for all slides in the presentation; there is no way to do this for individual slides. Here's what you need to do to change things up.

Go to Google Docs in a web browser. You can’t do this in an Android or iOS app.

Select menu (three lines).