To change a Smart Attachment that you sent in a message, follow the steps below.
Upload the new, corrected Smart Attachment. To Upload go to Content>Smart Attachments>New.
Delete the Smart Attachment you want to replace by selecting it in your Content Library then clicking on the X.
Go back to the new Smart Attachment and click Edit>Change the name to that of the original Smart Attachment that you just deleted.
The link in the message will now redirect to the new Smart Attachment.