Unfortunately there is not a way to edit these messages without interrupting delivery. To do this you will need to go to the message and choose "Return to Draft" Once your message is back in draft status you will be able to make changes to your content and then reschedule the messages to be sent out. If you expect to be making updates with some regularity, it may behoove you to put the content in a PageBuilder page as a reusable component and then put that component in the email's body. Then to make changes you need only update the PageBuilder page's content without changing the email message itself.