Hi, Mike,
There's no Undo function if that's what you're looking for.
When an order is completed, any items remaining on backorder are meant to be removed from Qty on PO.
While you could edit the Completed order and add a new item, if that's what you're really wanting to do, you wouldn't be unCompleting the prior items. You would only be changing the status of the existing order to Partially Shipped and the active item(s) would be any that you just added to the order.
If what you're wanting to do is to unComplete the items that were still on backorder prior to the order being prematurely Completed, perhaps the best thing to do would be to create a new order and re-add those backorders on to the new order.
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Regards,
Softrak Tech Support