You can make changes or revision before you pick up the ticket and if the reservation is prior to departure time from a PC or a smartphone. If you choose "Pay at station" as the method of payment, you cannot make any changes prior to pickup.
Once, you pick up the ticket, no changes can be made online.
* There are restrictions on changing reservations, such as regular tickets without special deals and some changes may not be allowed.
*If you wish to reserve a private compartment, you may not be able to add / subtract the number of passengers nor change from adult to child tickets, vice versa.