If you want to book your seating you can do it through two different methods, either on the web or over the phone -
Step 1: Go on the US Airways account via https://www.united.com/web/en-US/Default.aspx
Step 2: Select the flight you would like to edit seats with then click “select”
Step 3: Continue back to the seat assignment page then click the “View/Change Seat” under the listed passengers, then proceed to change your desired seat
Step 4: Continue on after you select your seat by clicking “complete seat reassignment”
or you can call here - https://gethuman.com/phone-number/United-Airlines
I hope this helps!