Voiding or reversing the original Invoice is the quickest way to issue a credit for a Prepay Invoice.
If this is not a feasible option, a new Invoice will need added using negative quantities. If the product was prepaid using a product specific Booking, then the Invoice will be automatically marked as prepaid. If the product was not from a product specific Booking then the prepaid Booking will need to be selected in the Booking Detail window during invoicing.