If you've scheduled an email and need to modify or cancel it:
To change the scheduled time
Click the dropdown arrow next to the "Scheduled" button
Select "Change schedule time"
Pick a new date and time in the modal
Click Save
To send immediately
Click the dropdown arrow next to the "Scheduled" button
Select "Send now"
To cancel the scheduled email
Click the dropdown arrow next to the "Scheduled" button
Select "Cancel schedule"
The email will return to draft status and won't be sent until you manually send or reschedule it.