You cannot change the SKU of a product already submitted to the Walmart marketplace. You need to make sure that you provide the correct SKUs for your products upon the first submission. However, if you need to make some adjustments, you need to recreate the item under the new SKU and resubmit it with SKU update set to yes.
Since this process is quite complicated, we recommend you contact our support team to perform the SKU update via the GeekSeller panel.