How to Modify the Master Slide in PowerPoint 2013
Make any formatting changes that you want.
Select the text you want to apply a new style to and make your formatting changes. If you want all the slide titles to be in italics, for example, select the title text and then press Ctrl+I or click the Italic button on the Formatting toolbar.
Make sure the Slide Master itself is selected — not one of its layouts. That way, any changes you make apply to all the layouts associated with the Slide Master.