If 發左貨, you can simply ask the seller to inform the 快運(at the departure city) to call 分撥中心 for the making change.
If it is already sent to 廣州, just ask the 快運 (at the departure city) to ask 快運 (at 廣州) to send back to 分撥中心 and then redirect it.
Anyway, just ask the seller to ask the 快運 (at the departure city) to do it for you.