Inventory cost should not be adjusted.  Inventory cost is what you actually pay for it.

Hello there, grace_yew,

You can directly go to the inventory item and edit the cost. Here's how:

1. Click the Gear icon.
2. Select Products and Services.
3. Locate the item, then click Edit under the ACTION column.
4. Scroll down to see the Cost box, enter the new amount.
5. Click Save and close.

Here's an article on how to use reports to see your sales and inventory status: Use reports to see your sales and inventory status.

If you have other concerns related to inventory, don't hesitate to post a question.