Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Manila to Changsha as you normally would.
These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Manila Ninoy Aquino Intl Airport to Changsha Huanghua Intl Airport flight would often be abbreviated as a MNL to CSX flight.