ebay won't allow you to change a return carrier

[Anonymous]

I have recently bought an item. It was not as specified so I opened a return for it. It answered me with a Royal Mail tracking number. Only trouble is the weight and size. It is to big and heavy to go with Royal Mail. It needs to be with Parcel Force. But the return won't let me change it. In fact I have got the seller to arrange a pick up on Friday, and he has provided me with a tracking number for Parcel Force. How do I change the tracking number and courier?

Thanks....Colin