The unit cost is set when a product master is created. The unit cost of the product master determines the unit cost for all its inventory.
Open the App Drawer and click Products.
Find the product in the list and click its Edit button.
Scroll to the Purchasing Information section. In the Cost Per GR or Cost Per EA field, enter the new cost. Do this for any additional vendors listed as well.
You can add new vendors and set the unit cost from them by clicking the Add Another Vendor (+) button.
See How to Create or Modify a Product Master for more information.