The following instructions are for: Responsive 6.0+, Mobilia 5.0+, Retina 4.0+, Parallax 3.0+, and Turbo 2.0+. If you're on an older/pre-sectioned version, please update your theme.
You can add custom content to the newsletter popup to encourage your shop's visitors to subscribe to your newsletter.
From your Shopify Admin, choose Online store > Themes > Customize.
Choose Theme settings > Popup/Newsletter.