Your theme determines the content that displays in your header and footer. You can edit certain aspects of both your header and your footer in your Site Designer app. To do so, go to your Site Designer app and choose Master Template from the dropdown at the top of the page.
From this point, locate the Header or Footer section and click the Edit Template button for either. On the left of the page, you should now see the block editing section for either the Header or Footer, depending on your choice. Specific options available may vary by theme.