Hello,.
Hello. how may I address you?
 I want to change my existing reservation. will you help me?
Sure, I am here to help you. Can you please help me out with your name?
 Sure, My name is Jonathan Green.
OK, Please stand by I will check and get back to you. Thanks for being patient. Unfortunately there is no reservation found against your name.
OK, Thank you for your information.
Thanks for reaching us, have a nice day.