You need to check with the seller of the ticket what the conditions are, thus if it is possible to make a change. 

As said it depends on the condition of the Seller and operator of the reserved trains.

I assume you have a reservation for the TGV Torino/Milano to Paris?
These reservations are limited and often sold out well in advance because of that i would be very carefull if you wanna cancel the reservations.

@seewulf They travel Rome to Milan. Is the reservation mandatory there? Otherwise it might be possible to travel without reservation. 

ohh sorry messed it somehow up But as written the rules of the seller and Trenitalia as operating traincompany applies.

Yes it is with Trenitalia. The reservations are mandatory as it is a fast train.

I suggest to try it at one of their ticket offices it´s the easiest way.