To change the slide orientation in Microsoft PowerPoint 2013, follow these steps:

1. Open your PowerPoint presentation in PowerPoint 2013.
2. Go to the “Design” tab in the ribbon at the top of the window.
3. In the “Customize” group, click on the “Slide Size” button. It should be located on the far right side of the ribbon.
4. A drop-down menu will appear with two options: “Standard (4:3)” and “Widescreen (16:9).” Choose the option that corresponds to the slide orientation you want to use. Selecting “Standard (4:3)” will set the slides to a landscape orientation, while selecting “Widescreen (16:9)” will set them to a widescreen landscape orientation.
5. If you want to set a custom slide size, click on the “Custom Slide Size” option at the bottom of the drop-down menu. This will open the “Slide Size” dialog box.
6. In the “Slide Size” dialog box, you can specify the width and height of the slides. You can choose either a portrait or landscape orientation by selecting the appropriate option under the “Slides” section.
7. After selecting the desired slide orientation and customizing the slide size if necessary, click the “OK” button to apply the changes to your PowerPoint presentation.

Your PowerPoint slides will now have the new orientation that you have selected.