add billing address after order is placed

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Hello - I have recently run into a situation where I need to be able to add a different billing address after the order has already been entered. I know how to change a billing address if it exists already in the order, but when there is no separate billing address at the time the order is completed, I can't figure out how to then ADD a billing address in the Shopify backend. When I pull up the order, I can see where it says that the billing address is the same as the shipping address, but I can't figure out how to change that so that I can enter a separate billing address. Perhaps (I hope) I am missing something very basic? Thank you in advance.