Open your Google Slides presentation and click on "File" in the top menu.

Select "Page setup" from the dropdown menu.

Click the dropdown box and choose "Custom" to enter custom dimensions.

Set the width smaller than the height (e.g., 8.5 x 11 inches for a vertical layout).

Click "Apply" to change the slide orientation.

Adjust text, images, and other elements to fit the new vertical format.

Review your slides to ensure proper alignment and formatting.