To change the shipping preference for already added products on Amazon as a seller, you can follow these steps:

1. Log in to your Amazon Seller Central account.
2. Navigate to the “Inventory” tab.
3. Select “Manage Inventory” from the drop-down menu.
4. Locate the product for which you want to change the shipping preference.
5. Click on the product’s SKU or title to access the product details page.
6. Scroll down to the “Offer” section and click on the “Edit” button next to the shipping preferences.
7. Update the shipping preferences according to your desired changes. This can include selecting different shipping options, adjusting shipping rates, or modifying the shipping settings.
8. Save the changes once you have made the necessary adjustments.

Please note that the specific steps may vary slightly depending on the version of Amazon Seller Central you are using. If you encounter any difficulties or have further questions, it is recommended to consult Amazon’s seller support or refer to their documentation for more detailed instructions tailored to your specific account interface.