Whether a reservation can be changed or refunded depends on the operator, the train and where you booked it. There are usually multiple websites where you can book a reservation. For reservations booked via Interrail, see this overview.
Italian domestic reservations can mostly be changed free of charge. They can be refunded if the price is higher than €10, with a 20% fee. You can exchange them in the Trenitalia app.
Eurostar reservations can be exchanged free of charge until 7 days before the travel date; less than 7 days before the travel date, there's an exchange fee of €15 (€20 in 1st class). a refund is possible until 7 days before the travel date, with an exchange fee of €15 (€20 in 1st class). No refunds less than 7 days before the travel date. You can choose a new date/time in the Eurostar app or on the Eurostar website, using "Manage your booking”.
TGV reservations can be exchanged free of charge. A full refund is also possible before the departure time. An exchange can be done in the TGV Inoui Pro app.