When you have all the slides you need for your presentation you will want to add a design feature to your slides. To view all of the designs PowerPoint offers. Click on Format on the main toolbar/Apply Design Templates. The design templates box will appear and if you scroll down the styles you should find one that you like the look of for your presentation. Choose a style and click on apply. All of your slides have taken on a lovely new look. To change a design at this point just repeat these steps. PowerPoint will automatically change the styles.

For a quicker way to get the design options and slides, click on Common Tasks on the toolbar, on the far right of the toolbar
.