Each airline has a different policy for the associated fees to make changes.
When changing the details of your trip, you need will need to contact the airline you are traveling.
Generally, any change to your original reservation (including flight, dates, or cities) is subject to a change fee per passenger (up to $250).
Additionally, the new itinerary will be priced at the lowest available published fare at the time of change, which may result in a fare increase.
For specifics about changing your trip, please contact the airline you are traveling.
Here are a few articles that reference change fees: