Labels can be deleted and updated in ReadyToShip but only before they are submitted to the carrier in a manifest. Changing manifested orders will usually require some interaction with the carrier outside of ReadyToShip.
If an order has not been manifested
Sometime when you are generating labels you will find one that can't be shipped.
Provided it hasn't been manifested yet, you can delete it by expanding the order and clicking the Delete Consignment button found at the bottom right of the order window.
For example in eParcel :
If an order has been manifested
Once an order has been manifested, ReadyToShip can't alter the manifest.
If you need to alter a manifested orders you should contact the carrier and ask them to adjust the manifest for you.
For carriers that don't have a Manifest step in ReadyToShip labels are usually generated in the carrier's labelling system outside of ReadyToShip. Either contact the carrier or log in to the carrier portal to see if you can remove or cancel the label there.