Tickets with the “Smart Light” or “Smart” fare are non-refundable: if you change your mind about flying, you cannot issue a refund. Such tickets can be exchanged through the Smartavia call center by paying the applicable fees. If you decide to exchange tickets, you need to notify the airline before departure that you are not flying, i.e. remove your seats from the flight.
For detailed return rules, see the section "Return and exchange of tickets".