To change the delivery date of a specific order, click "Manage Delivery" on the order you wish to update, and select "Change Delivery Date" from the menu that appears on the right.
If you're looking to change your default weekly delivery day, click Account, Account Settings, then Subscription and select a new weekly delivery day. Be sure to save your changes.
If you are looking to change your first order, please see: How do I reschedule my first delivery?