Hi, Fay.
Leon here from Shopify Support.
If an order has been fulfilled and delivered already, there is no way in the admin to go back and “re-fulfil” those items. In cases like this, you’ll need to create a draft order in the admin by going to Orders > Create Order, add the items that were missing from that orde, then buy a shipping label and fulfil the new order. In this new order, you can mark it as paid, discount it fully to not impact your sales figures, and add notes to indicate that the items in this order are a replacement for the previous. After this is completed, you’ll be able to look at the customer account in the admin and see both orders at a glance.
If you have any questions about this process, please let me know.
Have a great one,
Leon