My daughter booked us on a trip through United that flies on Air Canada from Denver to Toronto, and then on to Heathrow. It returns from Gatwick on Lufthansa, and then switches back to Air Canada in Toronto. She bought cheapest available, so I am sure they aren't easily flexible. First of all, who do I go through to try to change return times on flights? United? Air Canada? Lufthansa? i have never changed a flight time and am baffled where to start. I want to check on extending the time to a later date for two passengers of our four in London before returning, and if that isn't an option, she had scheduled a six hour layover in Toronto to see Niagara Falls, but did not take into account going through customs, etc. so we were wondering about moving our 6:30 flight later in the day or even until the next morning as it looks like that would not be enough time to get to the falls and back for check-in. Also, on the three hour check in on international, can that be done online or does it have to be at the airport if there is no checked luggage?. Any help is appreciated, as I am not sure where to start.