On Fulfill, open the batch and the order.
Choose Send back to Assign (remove from batch).
The order returns to Assign, ready to edit and re-batch.
⚠️ This voids the label. If a label was already printed, sending the order back to Assign voids it - you'll generate a new one when you re-batch. Repeatedly re-batching a labelled order can create duplicate labels and block that batch's manifest, so only do this after a printed label if you genuinely need to redo it.
Tip: to add a package to an already-labelled order, use Reship instead (keeps tracking on the original order).