You can use Purchase Orders to let the system know exactly how much you paid for each unit of a product.
However, if you need to make a change to your cost per unit for a product manually, you can do so by navigating to that product in Admin and viewing the Inventory tab for that product.
Hover your cursor over the average cost per unit field.
A purple edit button will appear. Click it to enter a cost per unit.
Once you've entered the correct value, click Save and you're done!
NOTE: When you change the average cost per unit, this will be reflected across the system. Current Inventory and Orders reports generated after changing the average cost per unit will reflect the new value.