As of May 2015, Cherbourg-Paris cannot be done entirely on a TER, but rather on an Intercités train. The fare conditions of a SNCF ticket (cardboard or e-ticket) are usually listed on the ticket.
For an Intercités ticket bought on voyages-sncf.com (tgv-europe.com no longer exists) on the Leisure fare for Cherbourg-Paris, it is possible to cancel it for free until the day before the departure, with a fee of 50% (max of 12 euros) on the day of departure. After departure, it is non-refundable.
Billet échangeable et remboursable sans frais jusqu'à la veille du départ, avec une retenue de 50% plafonnée à 12 € et réajustement au tarif en vigueur le jour du départ, non échangeable et non remboursable après départ.
In general, on SNCF tickets bought online at voyages-sncf.com or in a station or at a ticket machine on the Leisure fare, the conditions are similar: free for exchange and refund until the day before departure, exhangeable and refundable on the day of departure for a fee, and non-exchangeable and non-refundable after departure.
Prem's tickets are non-exchangeable and non-refundable.
To modify your ticket online, find your ticket reference number (6 alphanumeric characters), and go to the reservation manager.