If you want to create or design a new template, access your file manager, you can copy the template catalog/view/theme/default/template/extension/module/oc2/newsletter_popup_template_1.tpl and create a new template for example
catalog/view/theme/default/template/extension/module/oc2/newsletter_popup_template_2.tpl and make HTML modification as per your requirement.

For OpenCart 3.x.x.x users, the location is
catalog/view/theme/default/template/extension/module/oc3/

Once template is created, you can assign the template in the module setting.

If you want our support to create a customized template for you, contact our support via support center (Submit support ticket). The work will be billed at $7 to $10 per hour. For more information contact us via support ticket.