There are several reasons you may want to change your Google Slides from the default 16:9 horizontal aspect ratio into a vertical format.

For example, you decide to use Google Slides to design a PDF brochure.

Another example is that you want to give a presentation with slides that looks like a book’s pages.

You may want to showcase your portrait photography.

Change slides from a horizontal to a vertical orientation

Here are the steps for making slides vertical

Select File > Page setup from the menus
In the first Page setup dropdown list, select Custom
Change the dimensions in inches to 8.5 x 11
This is U.S. letter size, but you could use any other vertical dimension.
Click the Apply button
All of your slides in the current Google Slides file will now display in the U.S. letter size dimension.

Keep in mind that when you change the height and width of your slides, design elements will be automatically moved around in each slide.

You will need to drag text boxes, images, and other elements into the right places.