United gives you many options and one option is you can buy an additional seat if you are not feeling comfortable sitting in a single seat. But just remember that the additional seat will have to be purchased at the time of United Reservations. If you have not purchased it while doing your reservation then you can also book it at the time of flight departure. But the seat should be available for the purchase. You can book your seat into United first, United Business or Polaris class. To book an extra seat you can follow the instructions below:- • First open the official website of United Airlines and then go to the flight booking section. • Enter all the details related to your travel like departure time, departure date, number of travelers who are traveling, cabin class. • Now click on the search button and there is the number of United flights that will appear on your screen. • Now confirm the booking by making the payment done. You can also contact the United Airlines customer care number for any help related to this.