Just because you booked a round-trip flight to the United States with Singapore Airlines doesn't mean they have to send you on your way. If you realize your plans are going south and need to change your itinerary, there are a few ways of doing so. As Singapore AIrlines provides easy change flight policy.
If you realize that you may be too ill to fly or something has come up at work, or you just decide that you want to get home earlier than planned, don't worry -- change is possible.
To request a singapore airlines change flight contact the airline's office in your home country before your flight. You can reach the office by calling the relevant telephone number on their website; this is separate from their overseas call center phone number.