Hi Bunyah! Does this help?
You can request an upgrade when purchasing a ticket or after you’ve purchased your ticket. To request an upgrade when purchasing a ticket, either call United reservations or select the “MileagePlus Upgrade Award” option on the Advanced Search page on united.com. After you’ve purchased your ticket, you can request a MileagePlus Upgrade Award by calling United reservations, going to the Manage Reservations page, or talking to a United check-in counter representative or gate agent at the airport within 24 hours of departure.
The price of an upgrade is based on your region of travel, fare class and Premier® status. Check pages 4-5 of our Travel Award Destination chart for pricing details.