If a customer would like to adjust the tip on an order that hasn't been confirmed yet, you can use the Quick Edit feature as described here: Editing an existing order

If an order has already been closed and is no longer on the New Orders page, you can refund the order and reorder on the customer's behalf and add the tip. To go with this option, follow the instructions in this article: Reordering a past order on behalf of a customer

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