改航班时间要提前多少小时?英文怎么说?

假定你是乘客, 你的问题是: When is the deadline to change my flight?

假定你是职员,你对客户的回答是: You may change your flight up to XX hours prior to your departure.

the flight is put forward for ..... hours.

How many hours to be needed for changing a filght ?