Choosethe expressions can be used when you have to change the appointment or evencancel it.

A、I am really sorry to say, but I am afraid that I cannot make it on time,because … . Thanks for your understanding. ;
B、There is something unexpected happened, I have to stay to deal with it. Could I meet you at…?;
C、I will be out of town at that moment; I will be away for 6 days. I am afraid that we have to cancel the meeting. I am really sorry for it.;
D、I feel so sorry that I cannot meet you at… because… Can we change it for another time?