There is nothing you can do with your account itself to make the AP number appear automatically in all your bookings. Your UA number is hard-coded as your #1 number.
You are best to get the number changed by a human being. Either a check-in agent, gate agent, or RCC agent if there is a RCC at that location.
I don't have any current UA bookings, so I can't remember if there is anything you can do online, one booking at a time. But that will interfere with E+ seating assignments, if you are eligible for them. In any event I'd have it double checked by an agent at the airport.