What should be my strategy towards study for branch change from mining to CSE at IIT BHU?

• Every IIT has that handful of students who took up an average branch under the pretext of changing their branch in the future. If you are one of them and are seeking guidance in the same, this answer is for you. I won't mention any complex strategies and will stick to the basics
• Attend your classes: Many IITians underestimate lectures and tend to either skip them or pay no attention during classes. Trust me attending classes makes a whole lot of difference that can be felt during exams.
• Go to Tutorials: Students consider this childish and don't go to tutorials considering them useless. However, this will bring a big change too.
• Revise regularly: Lastly, whatever you study, revise it regularly or your efforts will go in vain. 

Many seniors may advise you that grades aren't that important, don't pay heed to them. Grades are very crucial for a branch manager don't let them make you believe otherwise. Lastly, be sincere towards your goal and stay committed to it.