how can i make Average Cost Update for past quanti

[rafeekg209]

how can i make Average Cost Update for past quantity
example in date 31-12-2008 i have quantity from item (A) 1000 PS and unit cost ber item is 1 pound then value = 1000 this item have transaction
since this date and quantity now = 500 ps i want today do average cost update
for quantity past = 1000 how ????