UPSC is a 3 stage   selection process involving Prelims, Mains and Interview which tests your personality test. Prelims has 2 Papers namely General Studies and CSAT.

General Studies has Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and Social Development, Environmental Ecology, Biodiversity and Climate Change and General Science.

CSAT has Comprehension, Interpersonal skills including communication skills, Logical Reasoning and Analytical ability, Decision making and problem solving, General Mental Ability, Basic numeracy and Data Interpretation.

Now for Mains, because it is an optional subject, you can choose among any of the optional subjects available. There is no criteria for selecting a certain optional subject. So even if you have done BA, you can still select Law as your Optional subject. You can select any subject that interests you or make you be acquainted with it. Remember that this subject should not bore you but make you think out of the box. And the books for preparation should be easily available. If Law has all these options for you, then you can certainly take up Law as an Optional Subject.