In general, flights are non-refundable.
If you would like to purchase a fully refundable ticket you will have to purchase directly from the airline and they are about 2x the cost of a non-refundable ticket.
When you purchase a non-refundable ticket, there is an option that airlines offer you, to cancel and or change if needed.
When canceling you have one year from the booking date to use the credit from the original ticket. When using that credit there is a fee – anywhere from $75-$150 depending on the airline.
You will then be responsible for the difference in airfare from the original to the new flight.
For specifics about changing your trip, please contact the airline you are traveling.
Here are a few articles that reference change fees: