I have a flight from Tokyo to Singapore and a connection from Singapore to Kolkata both on Singapore Airlines (both are in the same PNR/Booking reference btw). I am unable to change my itinerary on the website (it says unable to change as it contains partially flown segments). I wish to change the date of my flight (from 19th July to 24th July). How do I do it
Many flights, especially for more complex itineraries or if you are in the middle of your journey, cannot be changed online. Airline websites, if they allow changes at all, often only handle the simple cases; human intervention is required in more complex situations.
You'll need to contact the agent responsible for your booking. If you booked with a travel agent, you should contact them to change your flight. If you booked with Singapore Airlines directly, you should contact their customer service by telephone (that link will allow you to select your present country to get a local number for many parts of the world). The agent will be able to advise you of your options and price out the cost of any changes.