Average cost in DEAR is for information purposes only and is automatically calculated when any inventory movement (buy, sell, etc.) happens to the product.
Once On Hand quantity of the product becomes 0 (zero), DEAR stops updating average cost and leaves the last value intact. Then the only way to change it is via CSV file upload with any value you want.
In this case you need to do the update via CSV file if you would like to change current value (which is obviously not true average cost, but we receive inquires like this from time to time and the reason is to show margin %) or wait till you buy some new batch of this item and DEAR will recalculate average cost automatically once purchase invoice is authorized.
Average cost in DEAR is calculated as Value on Hand/Quantity on hand for every given product. But average cost doesn't affect your P&L statement because all COGS calculations done using FIFO method or Special methods. DEAR doesn't use average cost method for P&L calculations at all. Once authorized, invoice cost is available and distributed to all sales parcels (assuming you sold this product already) on FIFO (or Special) basic. No averaging used at all.