To upgrade with miles, you need an eligible paid ticket (basic economy usually doesn’t qualify). Log in to your AAdvantage account, find your trip, and check if a mileage upgrade is available. You’ll use a set number of miles plus a cash copay, depending on the route and fare class. If upgrade space isn’t open, you can request it and join a waitlist. If it clears, miles are deducted automatically. Availability is limited, especially on popular or international routes, so flexibility helps.