Irrespective of your qualification, Prelims, Mains and Personal Round of Interview are the only ways that some is selected for UPSC positions and in a way to become an IAS officer, that is the only route out. There is no short cut out there even if you have a MTech. CSAT is the 2nd paper of Prelims. UPSC Prelims has 2 papers. Paper 1 covers Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and   Social Development, General Issues on Environmental ecology, General Science.

Paper 2 covers Comprehension, Interpersonal skills including communication skills, logical reasoning and analytical ability, decision making and problem solving, General Mental Ability and basic numeracy numbers.

And clearing Prelims is important for entering Mains and Clearing Mains is mandatory for being selected for Personal Round of Interview.