Even if a shift is published or even booked, you can change the details of that shift. The employee who is booked for that shift, or the employees who are available for that shift will be notified of the changes instantly.
Start by finding the shift you want to edit on the web application scheduler and clicking it.
The edit shift screen will open, and you can edit any of the fields, then click update, and click 'Yes, update' to confirm
You can then confirm that your changes have been implemented by going back to the Calendar view