Sorry, I need to change plans.
'Programma' — pro-GRAM-ma. Double 'm' is clearly held. Stress on the second syllable.
A versatile, neutral phrase for changing or cancelling plans in any context. Less emotionally loaded than other phrases — clear and direct without being cold.
'Cambiare programma' (to change the plan/programme) is a standard Italian phrase for altering arrangements. It's slightly more formal than 'annullare' (to cancel) — it leaves room for modification rather than full cancellation.
Devo modificare i piani.
I need to modify the plans.
More formal — 'modificare' (to modify) is used in professional contexts too
Purtroppo i piani sono cambiati.
Unfortunately the plans have changed.
Passive construction — implies the change happened to you, not by choice
Dobbiamo rimandare.
We need to postpone.
Short and collaborative — uses 'we' which softens the message
In Italy it's very common to shift appointment times by an hour or two at relatively short notice. This flexibility (or lack of punctuality, depending on your perspective) is built into the social fabric.