When customizing your theme, by default, the social media links placed on the footer are populated with a placeholder URL so you can add your own links.
To add your own, navigate to the Website tab and click the Customize button.
On the navigation bar on the left, select Footer. Then, you will find the social media icons and links. replace those links with your own and click on Publish.
If you would like, you can also remove the social media icons on the footer.