Airlines make it clear when it comes to no show-up or cancellation, and most airlines only charge a fine for missing a flight if the passenger has the habit of doing it intentionally; this habit or practice is known as skip lagging in which the passenger makes the booking with no surety of taking the secondary legs of the trip to get the cheaper fare. So, a doubt arises whether to cancel or not show up. Then, the answer can be that if the passenger is unsure about the scheduled flight, it is better to cancel the flight than be a no-show. If you cancel, it might be possible that the passenger will receive partial or whole credit for the fare purchased, which can be used for future flight trips. Mostly used-based airlines charge around 200$ to upgrade to the introductory economy flight and that fare difference while rebooking to change the flight, but you will not receive a full refund.
If the passenger is late for the flight or misses it, the best way to tackle the situation with the airline is to call the customer service team immediately and inform them about the flight details. The earlier the airline knows about the issue, the more options will be available to fix the issue for less money.
Most airlines have an informal "flat tire rule" that the authority allows you to make a reservation on the next available flight ( without a hefty fee) if you can convince the airline to arrive late to board the flight. In the case of connecting flights, the airline will provide the ability during the fees until both legs are booked at one ticket and itinerary. In that case, the airline will reserve a seat for the next available flight.
You can cancel flight tickets through the web or contact the customer service team directly. Since web cancellation is easily accessible, here is the cancellation process that will make operations smoother.
If you cannot cancel the flight through the web, the respective airline team will give you the best instructions to cancel the flight. The team will remain active for 24 hours and seven days.
Yes, it is possible to skip the last leg of the flight, and it is not illegal, but make sure you have not checked baggage.