The only way it can be done is by scanning it at the airport or by taking a photo of it at checkin using the United app..then once it's done you won't have to do it again (in most cases).

I just got a new passport and I will just have to do that for my next international flight.. no big deal. Before you could do it on your phone it was a big deal if you were going for a day of departure upgrade and needed the time of check-in tie breaker. You can't get on the upgrade list until the passport is scanned. But the app works great.