This process will work whether or not the email has been opened.

To change an Image or Smart Attachment after a tracked message has been sent:

Go to the PoliteMail tab > Content > Images > Smart Attachments

smart attachments

Select the Image or Smart Attachment that needs to change and select Actions > Replace

replace

 

 

Note: This process is available for images and smart attachments. If you want to change a URL see this article.