It's going to change the font properties inside the entire box. In PowerPoint, you can either click and drag to highlight an entire block of text, you can click and drag to highlight everything, or you can just click once to select the entire text box. I'm going to change the font because it's tiresome to have to change the font every time I put in the new layout in my presentation. Let's start by making a change that will carry across every slide. Underneath this is every other single layout for that theme. And the very top one is the root slide that's used in all the layouts. I'm going to scroll all the way to the top.

To get into the slide master, change to the view ribbon tab and click slide master. You can change these by changing what's called the slide master and that's the master look of that particular slide for that theme. And that way, when you insert a new slide with the layout, it's going to look the way you want it to look.

You can manually override these every time with every slide, but sometimes it's easier just to change it all at once. Each layout and theme is predefined with a certain look, feel, color, even font properties.