Simply navigate to the flight tab on the sidebar and click the large “Change flight” button. Once you click “Change flight” you will be taken to the flight search results page, there you will see an edit button that resembles a pen and paper, on the top banner of the results page.
Clicking the edit button will open an “Edit Search” popup. In the right-hand portion of that pop-up, you will see a drop down menu listed under “Travelers/Class”, from there you can change your flight class. Once you have selected the new flight class, click “ Search” to see a new list of flights as well as the price per ticket for those flights.
Please note, changing the flight class of travelers for one city will not change the flight class of travelers for all cities in a multi-city itinerary.