To change the status of your scheduled email, navigate to your email (from the Calendar or your Emails sub-tab) to make the update.
Navigate to your email and change the status by clicking either Save as ready to be sent or Set back to in progress.
You can also change the status when editing your email's details by clicking
Save and Exit.
*Note: You can update an email ready to be sent back to "in progress" as long as you do so before its scheduled send date.