How, as an alumnus of IIT (BHU) Varanasi, can help our college become a better (hopefully the best) college?

If you are working in Google or Microsoft or any top tier companies or any company, as a matter of fact, you can conduct coding contests, you can invite the top three performers for internships or motivate them in some way. You can also conduct workshops and help students prepare for placements.

If you doing your Ph.D. in some foreign university, you can guide the students about how to go about research and the trend that is going on in the research field, so that the interested students can focus on the things necessary to achieve the same. You can also help in collaborating with your university with IIT BHU.

Other than that you can always contribute to alumni funds and help in placements and internships. You can also donate books to the library. If you are successful in your field you can hold webinars or seminars and motivate the students. If you have your own startup you can recruit students for your company rather than going for other IITs. This would also increase the placement percentage of the institute. You can also provide internships. You can support talented students belonging to poor families in any way that might help them.