Upgrade question for infrequent flyer

For someone who doesn't fly enough or is more interested in price/schedule than airline, how does the upgrade process work?

I'm flying on a 735 then 738, OMA-DEN-BWI. When I click through to the seat upgrade section, it says no options available for purchasing a FC upgrade.

Will this option be available within time? Or after checkin? Or would I have to inquire at the airport?