Courier and Service shown on orders

Hi, can anyone help please?

We have moved from Royal Mail to Whistl for our UK deliveries but the orders still show as Royal Mail when marked as dispatched.

This is causing us issues as the tracking number is Whistl which doesn't match the Royal Mail service stated and therefore it is affecting our VTR.

Does anyone know how we can change the courier and service to Whistl?

Is this set within Amazon when the order is created rather than being influenced within our internal systems?

Any help would be much appreciated.

Thank you.

Matthew