Sometimes, you must make a negative adjustment to the value (but not the quantity) of one or more items.
Open the Item Journal window.
Fill in the first journal line by decreasing the quantity on inventory with a negative adjustment. Use a unit cost that corresponds to the actual posted cost. For Help about a specific field, click the field and press F1.
Make a positive adjustment of the same quantity, setting the unit cost at the new, correct value.
You can find the actual cost on a ledger entry list for an item. If you set the filter Open = Yes, only the entries that are used to calculate the inventory value are shown. You can also see the cost in the Inventory Valuation report (under Inventory, Item Reports).