If you need to delete a scheduled post that has not yet been published, click into the pertinent location, then click the Posts option on the left side of the screen. Scroll down until you've found the post you'd like to delete, and click the blue Edit button on the far right side of it, as seen below.

When the edit post page loads, scroll to the bottom. You will see a red Delete button on the bottom-right, as seen below. Click it.

You will then be met by a pop-up that tests your resolve. If you are sure that you'd like to delete the post, click Ok.

Congratulations. Your post is now deleted. Please contact support if you have additional questions.