You can set the default shipping address as guided below.
【How to Change my Default Shipping Adress: Mobile Version】
(1) Finish logging into your account.
(2) Click the top left menu (≡) on the homepage's left sidebar.
(3) Click the "Account" tab bottom of the sidebar.
(4) Please click "My addresses."
(5) Click the "ADD A NEW ADDRESS" icon or click the "Edit" button to change the existing shipping address.
(6) You're all set.
【How to Change my Default Shipping Adress: PC Version】
Login > Account Page > My addresses > Default address > Edit