To edit the text version of your message, click on the Text tab in the message editor.
It is useful to create a text version for your email marketing campaigns because some recipients may not be able to view HTML. When you create a text version, they will see this version if HTML content cannot be displayed. Both the HTML and Text version of a multi-part email are not displayed at the same time - the recipient will see one version or the other based on their email program's capabilities and configuration.