Slide Master allows the user to edit the entire document’s design and the effects at one time. With slide master, you can set effects such as transitions for an entire PowerPoint presentation at once. To start, open PowerPoint, and select View on the ribbon. Under Master Views, select Slide Master. Now the style selections that you make will affect the entire document.
To change font, go to Home on the ribbon and change the font like normal. For transitions, click the Transitions tab on the ribbon, select a transitions, and click Apply to All. For animations, click the Animations tab on the ribbon and select the animation you would like to use. Closing Slide Master is as simple as navigating to the new Slide Master tab on the ribbon and clicking close on the upper right hand corner.
Knowledgebase Article: http://helpdesk.etown.edu/helpconsole2012/KB/default.aspx?pageid=slide_master