Selecting your branch of engineering involves social pressures in India… A lot of work that goes into robotics from a mechanical engineering perspective is multidisciplinary — drilling models, visualise how it works, calculation of velocity. There are also things like control systems etc.
You should also not worry too much about life-settling aspects of your choice. A lot of you have the option of charting your career. If you qualify for an IIT or an NIT the confidence you should gain is the ability to get through a competitive exam or a competitive situation. You will face a lot of competitive situations in the future. Don’t get unduly worried about the changes. The scope for lateral shift has improved dramatically.
As part of mechanical engineering, some of the institutes are offering robotics as a subject. Robotics is a real mix of all branches of engineering.
You need to be good at mechanical engineering, be a good control engineer, a good electronic engineer, any branch of engineering, gets into robotics. If one is really passionate about Robotics and you want to pursue that further to B.Tech. There are organizations that offer specialized courses in Robotics. Interested students can avail those facilities.
I have one suggestion: please don’t try to specialize too much at B.Tech level. At B.Tech level you please get your fundamentals clear in one area of engineering with a flavour of some of the other branches of engineering. There is much more scope for you at post graduate level to get into the specialization and maybe you should go into industry and come back for a Master’s degree.