Navigate to Email Center → Email Automation from the top menu
Click the name of the automation you'd like to stop.
Click the "Stop Automation" link in the top right corner of the individual automation screen.
Confirm you understand what will happen by stopping your automation, check the box and click the "Stop Automation" button.
The status of the automation and all the emails in the automation that have not yet sent change to "Stopped"
You may still view individual emails if you need to copy any content for new emails.