To edit a scheduled email prior to sending, cancel the send, edit the email and set up the schedule again.
Step 1: Navigate to the Main Menu of your account
Step 2: Click on 'Triage', you'll see a second menu appear showing options to choose from.
Step 3: Choose 'Scheduled' and find the email that you would like to edit.
Step 4: In the scheduled email you'll see the option to 'Cancel Send', click on that link, edit your email and re-schedule your email.