If you have stock in some item with the wrong cost, and already sold some of the item, refer to the example below to have a better understanding on how to correct it.
Example:
2009-01-02: stock in item and create GRN with cost RM13
2009-01-03: create invoice and sell the item
2009-01-05: realize the cost of the item is only RM10
* the same procedure can be use to correct the GRN with wrong date
To learn on how to create GRN, by create GRN by PO or Direct Receiving Stock.