Hello,

Maths is there only in the prelims exam, and that also part of qualifying paper CSAT for which syllabus consists of basic numeracy, comprehension, logical reasoning and analytical ability, general mental ability, decision making and problem solving, interpersonal skills including communication skills. So, CSAT paper is a mix of Reasoning, Maths, and RCs, maths here is of basic level only which we all have studied, you can use NCERTs books for your conceptual understanding, some of the other best options are M Tyra Quicker Maths, RK Agarwal's Quantitative Aptitude for Competitive Exam, Arihant Objective Mathematics. Prelims is all about speed coupled with knowledge, hence in this section what matters the most is practice, practice and practice, many of the aspirants who have great conceptual understanding don't go for such detailed study, what they usually do is apply many mock papers before the exam, but this case may not work for someone whose foundation isn't strong, so you have to see what works for you, other than prelims CSAT paper, maths doesn't have much importance in UPSC as it's more humanities oriented unless you take it as an optional subject.

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.

For Analytical Ability, Basic Numeracy and Problem Solving, you will need knowledge of Mathematics until 10th standard. So you may have to practise Mathematics until 10th standard. NCERT books might help you with this.