Kindly follow the steps below to set your default address:
Click 'My Account' at the top right of the page.
Select 'Account Details' in the dropdown.
Navigate to the 'Your Address Book' towards the bottom of the Account page.
Click 'Edit Addresses'
If you have multiple addresses in the Address Book, you can set one of the multiple addresses as 'Primary Billing' and/or 'Primary Shipping'