Please follow these step by step instructions to easily add / edit the SKUs of your product:
- Head to your WordPress administration Panel
- Go to WooCommerce > Products
- Search and select the product you would like to modify
- Under "Product Data", navigate to the "Inventory" section
- Add/Modify the SKU for the product and click the "Update" button on the right of the page to save the changes
Note: Product SKUs need to unique (different) across all listings. For variant products, the SKU for each variant option has to be different.
Video Tutorial: How to add SKU to the product in WooCommerce?