You can change a section layout in two ways:
From the section itself (recommended):
- Go to the section you want to update.
- Click Layouts (the button shown on the section).
- Browse the layout options and click a layout to apply it.
From the Pages list:
- In the editor, click Pages.
- Find the page/section you want to update.
- Click the Layouts (palette) icon next to it.
- Choose a layout from the list to apply it.