To resend a returned shipment, you'll need to delete the original shipping label from the system and create a second shipping label.
To delete the first label, click the order number, then click the blue Shipping Labels button at the bottom of the order detail screen. On the shipping label screen, click the Details hyperlink to the right (in the Labels column). Click the Force Delete button or the Delete or Void button - whichever you see - terminology may be different depending upon the carrier or point in time. This will delete the label and the tracking number, and will restore the order status to Unshipped.
Now you will be able to create a second label. Remember that if in Settings / Boolean Options, the Shipped Email is turned on, then when you create the second label the customer will be notified via email that the package has shipped or will ship (depending upon the date you put on the label).
Please note that changes to the order do not automatically make changes to any credit card processing that has already been done. If you have modified the order to add additional shipping charges or in any other way that increases the order total, you will then have to do the necessary credit card processing as well to charge the additional amount. This is accomplished in the credit card terminal accessed by clicking the blue "credit card terminal" button at the bottom of the order detail screen.