Want to cancel a scheduled campaign? Do it as soon as possible: once the campaign has been sent, it can no longer be cancelled or stopped.
How to unschedule a campaign
Go to Campaigns and open the scheduled campaign.
Click Edit.
Confirm: the campaign goes back to draft. It won't be sent until you submit it again.
Unsure about the content or the send time? Unschedule it as a precaution, fix it, then schedule it again. It's always better than an unwanted send.
To schedule it again afterwards: How to modify a campaign that is already scheduled?