If you want to change the Master SKU for a listing, follow these instructions:
Step 1: You will click on the cog under your title and existing Master SKU.
Step 2: Once on the Master Listing for the product, delete the existing Master SKU and input the updated SKU you will use to identify your product:
Step 3: Save your listing details.
Reminder: Listing Mirror will not change existing live marketplace SKUs. Changing your Master SKU is only changing the internal identifier for your product and will only affect listings created AFTER the change is made.