Open your Google Slides presentation.

Click on "File" in the top menu.

Select "Page setup" from the dropdown.

Click on the dropdown menu that shows "Widescreen 16:9".

Choose "Custom" and swap the width and height values (e.g., 10 x 5 inches → 5 x 10 inches) to rotate the slide.

Click "Apply", and your slides will switch to a rotated layout.