When you are in the weekly view, you'll see a cog icon next to each of the day names. You can click on that icon to bring up a menu of options for that day.
Select "Clear" and then you can choose if you want to "Bump Plans Back" to move your plans backwards.
If you clear a day accidentally, you can undo it by clicking the "Undo" link that appears at the top of the page (below the planbook title).
See our article on Bumping a Day's Plans- Backwards for more information.