Whether you're customizing a standard email or creating an email from a blank, you can easily alter the banner in your email. Follow these steps:
Below is an image from creating an email from blank. Simply click the upload
Browse to your banner and insert the image into the email
Proceed with creating the remainder of your email.
Note: If we had created one from a template, we'd see the default content, including the banner. So we might see something more like this:
Simply highlight the existing banner and delete it. Then upload a banner as you did above.