This article applies to Classic Commerce. (Looking for Contextual Commerce documentation?)
Once you have created an Email Fulfillment Template that is Reusable on Multiple Products, go to SpringBoard's Store Home » Products and Pages. Select a product that has the Email Fulfillment Template you want to change. Click to Edit the Email Fulfillment. Edit the contents and click Save. This will change the Email Fulfillment Template, which, in turn, will change the Email Fulfillment for any products that uses this template.