To delete scheduled posts, first make sure you select the Social Account, go to the Scheduled Posts section, and find the post under the Future Posts tab. Then, click on the Delete button (trash can icon) and confirm. If you're on the compact view, you'll need to click on the three dots menu first.
If you're looking for a way to delete many posts at once, another option is to click on the checkbox located at the top of the Future Posts tab. All the visible posts will be automatically selected, which will enable different bulk options, such as saving as draft, approving, or deleting posts. Select the posts you want and click on Delete (trash can icon).
An alternative option, instead of deleting posts you don't want to post, is to move them to Draft. Learn more about it here.