Hello there, @pial.

You can download a copy of our product and service items. Then, change the amount on the Purchase cost column. Once set, you can import it back to your QuickBooks account. I can guide you on how to do it.

1. Go to the Gear icon.
2. Choose Product and Services.
3. Click the Export to Excel icon. 

Next,  let's make the necessary changes. 

Now, you can import it into QuickBooks. Here's how:

1. Go to the Gear icon.
2. Choose Import Data.
3. Select the Product and Services tab.
4. Click the Browse button.
5. Locate the saved file then, click the Next button.
6. Map your excel fields to the QuickBooks fields then, click the Next button.
7. From the Import products and services page, review the item details.
8. Make sure to mark the Overwrite all values for each product or service that you import with the identical name tick box.
9. Click the Import button.

Just a heads-up any changes made is not retroactive to the transactions you've already created. You can read through this article for more detailed steps: Import products and services from Excel.

Get back to us here if you have other questions or concerns. I'm always here to help.