Orders in a shipping tab can be moved back to Unshipped Orders, but only if they haven't been manifested or completed.
The easiest way to remove an order from a shipping run is to click on the red arrow icon in the second last column:
If the order has a label that has already been printed, ReadyToShip will delete the label and move the order back to the Unshipped Order tab. From there, you will need to allocate your preferred carrier before moving into a shipping run again.