Southwest Airlines offers passengers to upgrade their boarding position to A1-A15 within 24 hours before the flight's scheduled departure, and the Southwest business select upgrade cost starts at $30/passenger, and it might vary depending on several factors, such as fare type and availability. Moreover, by upgrading seats to a higher class, one will get earlier access to the flight and overhead bin space. You can find more detailed information on this by going through the guidelines stated on the Southwest Airlines official website or by reading the blog from the provided link.