You can edit the content using the Drag'n'Drop Editor.
The Drag'n'Drop template editor lets you modify your content with content blocks.
Plain Text Email
In addition to your HTML-based email, you can specify text that recipients will receive if they don't have an email client that can handle HTML-based email. In both the Drag'n'Drop and HTML-based template editor, click on the Plain Text Tab to open the plain text email editor.
There are two ways you can enter plain text content for your email:
Import from HTML— you can select to have your plain-text content imported from your HTML content by clicking the Import from HTML Link.
Click the Wrap Long Lines link to automatically wrap the lines to the “Recommended Width” length.
Click the Next Step button to continue to step 5 which is covered in the support topic Email Campaign Step 5:Delivery