You can do it either way, and also anytime between booking and the day before your departure date (seats go to "Airport Control" 24 hours before flight). Just go to united.com and select "Itineraries" and then use the "Guest" feature and enter your Record Locater number (usually a combination of letters and numbers like MUV8GU) and your last name and it will pull up your booking. Then click the "Seat Assignment" link and you can view available seats on a seat map of each plane you will be flying, and select the ones you want for advance assignment.
But before you do that, I suggest you go to
www.seatguru.com and look up the specific United plane types you will be flying on to see which seats are considered great, good, or poor on each aircraft type.
You can of course get advance seat assignments the old fashioned way by calling United Reservations and doing it with a live person. But personally I prefer to see the seat map myself via the website, and then select my seats from there.
If you are a Premier or above level member of United's Mileage Plus program, you get to select seats in the "Economy Plus" area of the plane free of additional surcharge. These seat have a few extra inches of legroom. If you're not a Premier member...United will often sell you a seat in Economy Plus for a few extra bucks. Well worth it, IMHO...especially on long flights.
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