How to Update or Add a UPC to an individual product in Shopify

In Shopify click on Products in the left navigation then the individual product you want to add a UPC to.

Scroll to the Inventory section

Add the UPC and click Save at the bottom of the page

How to bulk add the UPC to your Products in Shopify using a csv file

You can bulk update your products in Shopify using the instructions on the Shopify site:

You can check GTIN numbers here