Select the "System Settings" tab on the left side of the page then the "Network" tab at the top. From here, select a network interface, click the arrow next to the options button, and click "Edit".
This brings up the "Interface edit" menu, allowing you to change the network interface addresses.
Choose either "Cancel" or "Apply", navigate to the "Default gateway" section and click the "Change" button. In this menu, you can select your default gateway of all your available interfaces. Select one and click "Apply".
Under the "Settings" heading, you can now edit the DNS settings. When finished, be sure to click "Apply".