All tickets can be changed to another day free of charge at the moment. If you want to go ahead with claiming the taxes, you can use the online customer service form, but bear in mind. There might be a service fee which is higher than the taxes you will get back.

If the ticket is booked thorough a TA/OTA, you need to contact them, not SK. That also goes for the rebooking to another date.

Also remember that SK do not cancel the return flight automatically. You therefore have to wait till the return flights has passed too or claim taxes twice if you want to claim the unused taxes instead of changing the flight to another date.