Here's how to make a change to an email after you've saved it. You can make any changes to an email as long as the time it was scheduled to send has not already passed.
Navigate to Email Center → Email Automation from the top menu
Click the name of the automation that contains the email you'd like to edit.
Click the "Edit" icon next to the email you'd like to edit.
Make any changes you need, review the changes and click the "Save Email" button.