Step 1: Open your calendar by selecting "Calendar + Media" in the left sidebar.

Step 2: From your calendar view, click on the post you want to unscheduled.

Step 3: When the post opens up, click on the "unschedule" button. Then confirm that you want to unscheduled it. It will open the post in your post builder.

Step 4: From here if you want to delete the post, simply clear the post builder by clicking "clear all". If you want to edit the post and re-schedule it, make changes, then select a date and time, hit the "schedule" button, and you're done!