Depending on what you want to do, you can find slide layouts in one of two places. See below for details:
To pick a different layout for a slide:
In Normal view, on the Home tab, click Layout.
Pick a layout that best suits the content in your slide.
To customize or make changes to the placeholders and other objects that make up a :
On the View tab, click Slide Master.
The slide layouts appear as thumbnails in the left pane below the slide master, as shown below.
Click a layout in the thumbnail pane, and then begin customizing.