Dear Umair, if you want to pursue your 12th in Arts again, NIOS is the best option where you can study subjects of your choice and it us valid and recognised. However if you want to pursue Arts courses in your Bachelors, you can do that too. For example courses like BA (Hons) Psychology, BA (Hons) English, etc. You can complete your graduation in these subjects even with your science background. Both these ways, you will not be wasting any accademic year. 

Since you are now interested in Civil services exams, so arts will help you much. But this should not be the way to change your interests suddenly. Think properly first, if you want to crack civil service exams and you are 100%sure about it. Then go for arts, choose any hons subject according to your interest. Prepare for civil service exams but your decision should be final related to subjects choice.