When you click on make reservations, click on "Advanced Mode". At the very bottom, you will see a check box called " Mileage Plus Upgrade Award", Click that and do your search, you will see the flights that have seats available for upgrade with miles+ coppay

Additionally remember, the higher the fare class, the lower the mileage and copay requirement. With W, you are looking at 20K miles Internationally and 550$ co pay and I believe 20K miles and 75$ copay domestically