Hello Manas!

Material science is a really good choice at IIT Mumbai. The total fee of the course for a duration of 4 years is going to be INR 8.46 lakhs.

I am providing you with a link here where you can get further information regarding it.

https://www.careers360.com/university/indian-institute-of-technology-bombay/courses

Coming to your question about changing your branch in the second year, you can do that but that depends on a number of factors. Firstly, you'll have to maintain a really good cgpa throughout your first year and secondly it depends on the number of seats available in the following year.

So yes there is a fair chance of changing your branch.

Regarding the placements, they are pretty good too considering it's an IIT. For further information you should refer the above given link.

Have a good day