To get CSE in IIT Bombay, you need a JEE Advanced rank of 60 if you belong to General Category. This rank keeps getting lower every year. If you are certain of studying CSE at IIT Bombay, then start studying without further hesitation. In case you are still not sure, I have a friend's experience that might help you make the decision. He was interested in Mechanical and was not sure about CSE. With a rank of 70, he went for Mechanical Engineering. 

Now there is a course at IIT Bombay that is compulsory for all freshers. The course comprises the fundamentals of Computer Science. Once he studied that, he became certain that he wanted to pursue CSE instead and requested a branch change. In fact, I know many who changed their branch after becoming aware of their passion and inclinations. So you don’t have to necessarily make a decision right now, and can always avail of the branch change option later on. But make sure to make an early decision. Also if your JEE rank is average then you can always change your branch later on. However, an important thing to keep in mind is that this branch change option is possible only if your first-year CGPA is up to the mark.