If you've scheduled a post and need to make edits to the design itself, you can do so by making edits via your Order History. In the Order History, click the 'Make a correction' link, then make changes to your design on the editor page. Once done, click 'save and recreate order' to update your order.
The changes will now reflect on the scheduled post.
Changing your post caption
If you don't want to change the post itself, but instead want to alter the caption, this can be done via the My Posts tab in the My Stuff page. Click the down facing arrow on your scheduled post to reveal a set of options. Click 'edit post' to change the post caption, publishing page and time.