Email templates for the system and each module are located under the System Settings page. This is found under your profile picture / avatar (top right corner).
By clicking on each module under the Module Settings section, this will bring up the available options. Expand the "Email Templates" tab/accordion to view the templates.
Under actions, click the Edit button to change the template. You can change the content text, the button text, and the footer image. Simply hit Submit when you're done and the template will be updated.
If a module doesn't have the Email Templates tab, this means there are no emails that get sent for this module.