You might need to cancel a mailing that's been scheduled in order to make changes to it or change the time it will send.
Go to your Campaigns page, and click the Scheduled tab. This tab will only appear if you have mailings scheduled to send in the future.
Click the downward-pointing arrow to the right of that mailing you want to cancel, and select Cancel this mailing.
Click that, and a popup box will ask if you're sure, and you'll click Yes, I'm sure.
When you cancel the scheduled mailing, it will move back to the Unsent tab of your Campaigns page. If this was the only mailing that was scheduled, the Scheduled tab will disappear.