To change the design structure of your page first navigate to the Page settings editor.
You will notice that before each page there is a colored square. This square is here to make it easier for you to see what pages belong to the same design structure.
First, select the page you want to edit. After doing so, you will see the current design structure of the page inside the Design Structure field. To change the design structure, select another design structure from the list, or choose to Add a new design structure.
The colored square before the page name will change its color after a new design structure has been selected.
To learn more about the importance of design structures and how to use them, visit the following link.