The messages you have scheduled are shown in dark grey boxes on the campaign page. You can delete the post you scheduled by clicking ‘delete this post’ as shown below. After that, schedule a new post.
If you can’t see your message on the campaign page please read ‘How do I find my the post I scheduled?‘.
You can’t delete posts that were already published by the campaign manager. To delete message that were already published, go to your Facebook or Twitter account and delete it from there.