I need to change my reservation.

Meaning

I must modify my existing reservation.

Examples

• John: I've just realised I booked the wrong dates for our flight.
• Alice: Oh dear! <b>I need to change my reservation</b> too then.
• John: Let's call the airline immediately.
• Alice: Yes, we should do it soon; <b>I need to change my reservation</b> before the flight is fully booked.