In your HubSpot account, navigate to Marketing > Email.
Hover over the email you to edit, and click Edit.
In the editor, click the default text to open the editing options for the View As Web Page module.
On the sidebar pane, click Edit in template below Web version content and stylesto open the template.
If this template is being used for other emails, you'll be prompted to Cloned & edit new template. If you don't want your changes to affect other emails using the same template, you should select this option and save your new template in a file.
In the template, click the View as web page module to select it in the inspector. Then, hover over the default content and click the expand iconexpand to open the copy in a rich text editor.