If you have selected your seats in the basic cabin while traveling with Delta and want to upgrade it now, Delta Airlines Seat Upgrade helps you. Upgrade your seat with miles on the official website of the airline. Alternatively, you can upgrade your seat through the Fly Delta app. If you can’t do so, count on Delta Reservations for prompt assistance. 

You can upgrade your seats even after the check-in window is closed or 3 hours before the flight’s scheduled departure.