If a receipt note was updated by the Advanced Shipping Note (it will say ‘updated by ASN’ in the receipt line note), the items short will automatically be removed from the despatch note and go into Receipt Avail Lines to be receipted in later.
If you would like to reorder the quantity that is short, you can just delete the line twice off the linked PO – the quantity expected in will be attached to a receipt note so will not be deleted but the quantity that was short will go back onto the POP requirements list to be reordered.