Booked award reservation thru aeroplan - how to move to united.com?
You'll need to find out the United PNR. Once you have it, you can add it to your reservations in the app or through your profile on the website.
You can get the PNR by calling Aeroplan or UA - UA should be able to look you up by flight/name. Once you have hat, you can have a agent assign you a seat or you can do it on the website.