For the OP, on the App, once you're in your reservation,under 'next steps', click 'view all', scroll down to 'cruise options' and the first thing is 'dining'-it says what your current dining time is and there are buttons that say 'keep' or 'change'. Click 'change', and the options are main or second. If it is full, it won't let you click the 'main' button and says 'we're sorry, but the seating you selected is currently unavailable...'