German phrase
Ich möchte ~ stornieren.
Meaning
I would like to cancel ___
When to use it
This requests a cancellation. It is polite and clear when cancelling a booking or purchase.
Example dialogue
Hallo, ich rufe wegen meiner Buchung an.
Hello, I'm calling about my booking.
Ich möchte meinen Flug stornieren.
I would like to cancel my flight.