Hey

A science student can switch his field of d=study after completing a full time program, so you can definitely change your stream to Humanities/Arts after class 12th.

And to join Indian Army as a Female Officer you must look up to following possibilities:

1. Service Selection Board (SSB): Assuming that you would be applying after pursuing graduation in Arts then SSB is the best option for joining Indian Army. It consists of a written paper conducted twice a year and followed by interview process
2. National Cadet Corps: You can join NCC in your college and volunteer for the same, later there on there are recruitment from NCC camp through which you can be the part of Indian Army

If interested, follow the link for more details; SSB