To change the default shipping address for all of your orders placed through Portal, navigate to your Orders page. Open any order draft and scroll to the Shipping & Payment section. Select the pencil icon to switch to edit mode, then the select the Change button in the bottom left. corner Within the Address Book window, select the shipping address of your preference. This is shown below for reference.
Select the existing address you would like to make your default or add a new address using the Add New Address button. Hover over an existing address in the Addresses list and select the Set as Default link as shown above.
NOTE: This will change the address for all future orders, but it is not retroactive for any PO's that were already created.