That's a great question! I can understand how helpful it would be to edit your email once it has been sent. However I apologize this is not an available feature. One an email has been sent you will be unable to edit any of the information in it including its links. Instead the campaign will need to be copied, updated, and resent. Being able to edit or swap out URLs in a sent email is a feature request we have tracked in your account in the meantime.