ウエッブサイトで予約した航空券のフライトを変更したいのですが変更できません。どのようにしたらよいのか、また追加代金がかかるかについて問い合わせたい。

To ABC Airline Booking Office:

My name is [your name].
I have booked a flight with your airline through your website and would now like to change my flight. However, I was unable to do so online.

The flight information is as follows:
Reservation made on [January 3, 2012]
Booking number: 1234567
February. 25 UA 804 departing at 16:00 from Tokyo Narita to Washington Dulles

I would like to change my reservation to a later flight on the same day, i.e. the flight on February 25, 2012 UA 806 departing 20:00 from Tokyo Narita to Washington Dulles.

Please let me know how to make these changes online through the ABC Airline website and if there will be any additional charges.

I look forward to hearing from you.
[your name]