All of the default social media links go to Shopify. This is how you change them:
1. On your dashboard menu (left hand side of the page) click on "themes" - This will bring up your current theme in the main window.
2. Click on "Customize Theme" in the main window. This will bring up your main theme editor. You will see the main window now on the left side of the screen and your option list to customize on the right side.
3. Locate where on your page your social media links are. They will probably be in the "header" "footer" or "sidebar" - Clicking on any one of these titles in the menu on the right side of the page will bring up the different options for that area. You are looking for a section called "Social Media Buttons"
4. Under "Social Media Buttons" there are places to insert the URL links to your social media, your facebook page, twitter etc. These will have Shopify links in them. Delete the Shopify link and inset your links.
5. Save your changes - and your done!
Hope this helps!