Ticket Change Date

I bought my flight in advance, but i made all my visa process, it seems i have to go out earlier of the country (Thaialnd), (before my date-flight). in the conditions of my ticket i can not change the date, however i would like to know how many do i have to pay in order to change this one, i woulnt like to cancel the flight!!!!! . I really hope you can help because if i stay more of the time assigned, i could been in trouble.