Basics

MicroBiz Cloud uses Perpetual moving average cost to track the value of your inventory.
Supplier Shipment Cost, Store Transfer In and Beginning Stock Balance cost are all used to calculate the value of your stock - this is tracked for each simple SKU for each store. (Summary valuations for the configurable parent are shown in the inventory tab).
Transfers Out, Inventory Adjustments and Cost of Goods for sales use the average cost at the time of the transaction for that SKU/Store.

How Perpetual Moving Average Works:

If you have 6 items left in stock that you had purchased for 37.50 your inventory value = $225.00
Even if your replacement cost for a new shipment is 38.00 if you sell one your cost of goods will be 37.50.
If you receive 12 more at a cost of 38.00 you now have 18 at a cost of $456.00.
The total value of your inventory for this item is $225.00 + $456.00 which equals $681.00
The average cost of your existing stock is now $37.83 ($681.00 divided by 18 pieces).

If there is no inventory or only an adjustment in - since there is no existing average cost we use whatever the replacement cost is at the time of the transaction.
Note that changing the replacement cost does not change the cost in an existing transaction.

You can edit completed vendor shipments if the cost on the invoice is not what you expected when you received the shipment - however existing transactions subsequent to the shipment are not corrected for this change (on the future features list).

How Can I change the Current Average Cost if I think its wrong?

How Can I change Cost of Goods on an Existing Sale?

The only way is to return the sale after the average cost has been corrected and resell it - but the sale will move to a different date.