Changing your flight ticket
If you have a booking agent (when you book through a website other than the airline itself), you will most likely have to pay a fee for changing the ticket as well. This fee is often around $20 or $30 but can be higher. Most agents have the option to freely change your ticket when you pay them a small fee (usually between $10 and $25) while booking the ticket. HOWEVER, you will still have to pay the airline fees for changing a cheap economy ticket ($150+ and changes in airfare).
Before your first flight, you will have to change your flight through the booking agent. After your first flight, you can call the airline itself to change your ticket for your remaining flights. When you call the airline, they will usually give you the option to change your flight right away. Sometimes, they will call you back because another department has to calculate the extra charges. If you want to confirm the change, you will have to pay immediately by using a creditcard.
If the airline office charges 150 euros for changing your flight, it can help to call the American office, that will charge 150 dollars instead. Depending on the exchange rate, this can save you some money. Most offices can be reached 24/7.