Hawaiian airline is quite flexible with its customers in terms of a flight change or any other alteration. If you have booked the ticket and you want to lock a  particular seat for yourself then the airline also gives you such sort of facility. Once you are done with your Hawaiian Airlines Reservations and you are all set to go, then 24 hours or 2-3 days ago before the departure of the scheduled flight you can reserve a seat. For doing so you have three ways –
Contact the live representative,
Reserve a seat for yourself directly from the airport
Or simply go to manage my trip option