Log on to the website where you purchased the tickets.
Click on "My trips," "Itinerary" or similar wording, depending on what travel agency or airline website you visit.
Click the "Change or cancel tickets" button or anything similarly worded. Follow the onscreen instructions to change or cancel your tickets, or call the number of the travel agency or airline company. The process will vary with each travel agency or airline website. For example, Orbitz has a step-by-step guide to help you change or exchange tickets, while Priceline has a contact number you must call, and United Airlines has both features you can use.