Getting a "free" upgrade on United Airlines almost always requires Premier Elite Status (Silver, Gold, Platinum, or 1K). For domestic flights, Premier members are automatically placed on a "Complimentary Premier Upgrade" (CPU) list, which clears in order of status level and then fare class, starting as early as 96 hours before departure. If you don't have status, you can use MileagePlus miles plus a potential cash co-pay to "request" an upgrade, though this isn't strictly free. Another strategy is to hold a United co-branded credit card (like the Quest or Club Infinite), which can move you higher up the standby list or allow for upgrades on "award tickets" (flights booked with miles), a perk usually reserved for higher-tier elites. On rare occasions, if a flight is overbooked in Economy but has open seats in First, a gate agent may perform an "Operational Upgrade" (OpUp), usually selecting a solo traveler with high status. To maximize your chances, always check in exactly 24 hours before the flight and monitor the "Upgrade List" in the United mobile app.