To change an optional subject after applying for the UPSC exam, candidates must follow specific procedures outlined in the UPSC notification. Changes are typically permitted only during the application stage before the submission deadline. Aspirants need to log into their portal, navigate to their application form, and select the new optional subject from the available options. It's essential to ensure that the new subject aligns with their study preferences and strengths, as this decision can significantly impact their overall performance in the examination.