For security reasons and to avoid potential fraud, we restrict signup to the email address used when you signed up with Shopify. 

If you change the email address that you signed up for Shopify with, it should update on our side. 

Details on how to do this in your Shopify Account can be found here: https://help.shopify.com/en/manual/your-account/manage-account#change-your-store-owner-account-email