What to do in this situations:

Already booked my flight and I would like to change the departure date. Where can I do this?

I would like to book online a ticket for a under age child that travelers alone; what are the necessary formalities?

I booked a flight and I am unable to take the trip. Whom can I address to, to change the reservation name to another passenger?

I booked a flight and I want to use only the return part of the ticket, valid for the return flight. Where do I have to announce this option?

I booked a flight but I'm unable to take the trip. Whom can I address to to cancel the ticket and have the reservation sum returned?