When a credit note is created for an item, the Unit Cost is viewable and editable on the item detail screen. It contains the unit cost that was posted with the invoice (as typically this should net to zero after crediting).

I assume that you are wishing to edit the unit cost from the costed amount from the invoice to something new. As this desired value likely isn't stored anywhere within the database, the solution is to edit the Unit Cost while entering the credit note to reflect restocking fees, etc. Then OE posting and Day End will apply the appropriate costs.

Accpac Plus OE does the same thing...