You can only edit shipments in "order state," meaning pre-shipment generation. This article will outline how to edit synced orders.
It is also possible to Bulk Edit Shipments for specific properties, see our article here.
Navigate to Create Shipments > Advanced and locate the order you wish to edit.
Edit Order Details
Click on the first to edit order details.
The order profile will pop up, and you can edit it freely. Make sure to Save once complete.
Edit Shipping Address
Click on the second box to edit the shipping address. You can edit the destination and sender address.
Once changes are made, click Save.
Select a New Courier
Click on the third box to edit the selected courier.
Select the courier, and click Save.