I purchased a train ticket from Cherbourg to Reims from TGV Europe. This was two trains, a TER service from Cherbourg to Paris, and a double-decker TGV from Paris to Reims. The TGV part was a "prem", and if I cancel the ticket I'll lose money. But that's okay, because I don't want to cancel that ticket: I still want the TGV from Paris to Reims, but I no longer want the TER from Cherbourg to Paris.
Is there any way I can cancel only the first part of the ticket? I cannot find this option on the website, and they don't provide contact details anywhere obvious.