To clear your schedule, simply click on the schedule tab from the left hand menu.
Once in your schedule, above your calendar you will see a 'clear schedule' button. Use this button to delete all posts from your calendar and start from scratch.
Don't forget, this will remove all posts and cannot be undone.
If you want to remove just one post or a post repeated weekly/monthly, click on this post in the calendar and click on the pencil icon in the post preview on the right. Here you can choose to delete (for just that one post), or delete all (to delete the series).