Delta airlines (delta.com) rewards its frequent flyers with free and unlimited first-class upgrades. Sign up for a rewards membership with your credit card, travel as you normally would and you will eventually earn enough miles to achieve silver medallion status, which starts the free upgrades. Complimentary upgrades are subject to seat availability. ‌Keep wracking up delta skymiles to achieve progressively higher upgrade priority as a gold, platinum or diamond medallion status member.‌

Go to Delta's website to sign up for a free Skymiles account. Go to the "Skymiles" tab in the upper right corner, scroll down and click "Join Skymiles." Fill out the electronic application for delta one.

Travel on Delta or one of 16 partner airlines, such as Alaska Air, Air France, Aeromexico, and Virgin Atlantic, to collect medallion qualification miles, or MQMs. Provide your Skymiles number when booking delta flights or during check-in, or sign into your Skymiles account and book online through Delta, then your round-trip flight and economy ticket will be visible in my trips, along with available perks and upgrade list for skymiles members and elite members. This can also be visible in the delta app. MQMs are calculated by fare type and flight length.

Achieve silver medallion status at 25,000 MQMs. Silver medallion status gives free upgrades, but higher status members enjoy more benefits and more delta upgrades. Gold medallion status is at 50,000 MQMs, platinum at 75,000 and 125,000 gets you diamond status.

‌Make a reservation to fly Delta and provide your medallion status Skymiles number.‌ Delta will automatically upgrade your fare class to first class or business class from the main cabin or basic economy for free if a first class seat is available and you can fly in delta comfort. No need to make an upgrade request for a seat upgrade. Silver, gold, platinum and diamond medallion members are given progressively higher seating priority for eligible flights along with elite status, even on international flights not just domestic flights.

Regional upgrade certificates, global upgrade certificates, and companion certificates can’t usually be done last minute on the day of departure if you’re trying to upgrade your economy fare to the premium cabin with select seats. However, premium economy might be easier to achieve if you are a platinum medallion member.