Why?  It's automatically calculated by the system with each transaction.  Anything you put in there would be overwritten very soon afterwards.  If it's wrong, correct the warehouse amounts (quantity / cost), and CI_Item would be updated by that adjustment.

Average Cost is recalculated when you add inventory quantity.  If you have quantity on hand right now, you need to remove all of the inventory, and then add it back in at the cost you want it to be.  For example, export the quantity on hand by item and warehouse, do a physical count to set all counts to zero, then update the journal.  This will set all QOH to zero, but you may still see an average cost on the Item master.  Next, import or enter the correct quantities with their correct costs in an Inventory Adjustment.  When you update this journal, it will reset the Average costs to cost entered.  

Average Cost is recalculated when you add inventory quantity.  If you have quantity on hand right now, you need to remove all of the inventory, and then add it back in at the cost you want it to be.  For example, export the quantity on hand by item and warehouse, do a physical count to set all counts to zero, then update the journal.  This will set all QOH to zero, but you may still see an average cost on the Item master.  Next, import or enter the correct quantities with their correct costs in an Inventory Adjustment.  When you update this journal, it will reset the Average costs to cost entered.  

Thnaks, I'll try it...