Singapore Airlines' cancellation policy varies depending on your ticket type, the time of cancellation, and where you booked your flight. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) For specific details, you should always review the fare conditions associated with your booking. 

Within 24 hours of booking

• Flights to or from the U.S.: You can cancel without penalty for a full refund within 24 hours of booking, provided your reservation was made at least one week before the flight's departure. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) This applies to all fare types.
• All other flights: The fare rules for your specific ticket will apply, even within 24 hours of booking.