I booked a flight with my 16 yo offspring and my wife. I later cancelled my wife off the reservation. With just one traveler (not in the UAM age range), I tried to change the seat by clicking on the listed seat assignment on the reservation in "my trips". Errors arose. "Seat assignments not available - unaccompanied minors are assigned seats by blah blah blah" whatever. I thought maybe UA changed the UAM policy. After much fiddling around, including an attempt to use the online chat to rectify the situation to say "how dare you!!", I just clicked on the "view seats" icon on the bottom right. From there, I was able to select a different seat with no problems.