It's easy. We actually had to reschedule twice. My wife went on holidays and we were first rescheduled the day of her flight so we sent a letter to IRCC office explaining why and they got back to us with another date but the date that was given us was wrong because my wife would still be on holidays and they responded right away that we will be rescheduled when she's back. We got our 3rd appointment a week before she was back.
So basically the reason must be valid and send a letter not informing them that you wil not be there (it shouldn't sound or look like your PR is not important) instead ask them if they would grant your request.