Singapore 1-８３３－４４４－７６１３. Airlines allows cancellations with full refunds within 24 hours of booking, provided your departure is at least two days away +1-８３３－４４４－７６１３ or 1-８３３－４４４－７６１３ (UK).
https://www.devex.com/people/how-late-can-i-cancel-a-flight-in-singapore-airlines-2214208
