Using the Full Item Spec

When you set up your items, the item's Product Identifier (the combination of Product ID Type and Product ID) is associated with a single SKU. You are not allowed to submit two SKUs with the same Product Identifier. Even if you have submitted an item with a group of Product Identifiers (e.g., an ISBN and GTIN), this group can only have a single SKU.
If you would like to change this SKU, you can update it as follows:

1. Open the Item Setup template that contains the incorrect SKU.
2. Enter the correct SKU for that Product ID. 
3. Enter Yes in the SKU Update column, which is in the Nice to Have group. 
4. Upload your template file. 
5. Once the template is processed by the system with no errors, your item will be saved with the new SKU. The item will retain all of its ratings and reviews.

API

For details on how to update a SKU via the API, refer to the steps listed under Bulk create/update items in the API Documentation.