HI.
Hello. What help that I can assist you?
I want to change an existing  reservation due to sudden change in my trip. Can you help me?
Sure, I can help you and can you please mention your name for checking purpose?
Paul Collins.
Please wait for a moment.
Ok.
Thank you for waiting, I have checked in reservation list to change your ticket but could not find any reservation with your name.
No issue. Thank you.
Thank you for reaching us, have a nice day.