In order to change the overall layout for a PowerPoint 365 presentation, follow these steps.
Go to View > click on “Slide Master”.
From there, you should see a big list of files on the left side.
Notice that some of those slides are bigger than the rest.
Click on a slide that looks bigger. You should be able to edit elements there, graphcis, images, backgrounds, even text.
This is also very helpful if you want to transfer things like images from a PowerPoint 365 presentation to another.
Just select an image, copy with Control+C and then Control+V on the other PowerPoint presentation.
Once you are done editing, click on the button called “Close Master View”.
After you close the Master View, you will see your normal PowerPoint presentation with the updated layout.