Short term

Hello Kaushik,
Short terms courses should be in such a way that you learn something useful for your undergraduate degree.
As you are going to pursue B.Tech computer science, it is better to learn the programming languages as they form the basics of all software and object oriented programming.
This will be easy to learn in this short time also. Then, you should get updated with the latest technologies and those will not be taught to you.
Computer science is a competitive course so you need to have lot of knowledge to be successful and try to be better than your counterparts.
All the best!

Some of the good institutes are as follows:
MAAC, LISAA School of Design, Graphic and Design Institute.

There are many short term courses like Tally which you can do with Bachelor degree you can do diploma in taxation management foreign exchange. You can do diploma in import and export diploma in auditing is also there. These all courses are for one year and Tally is for 6 to 7 months.

Srimati Techno Institute | Hazra, Kolkata
Vision Institute of Professional Studies (VIPS - Kolkata) | Santoshpur, Kolkata
Kohinoor Technical Institute | Ultadanga, Kolkata
Exterior Interior Limited | Kalighat, Kolkata
WLC College India, Kolkata | Salt Lake City, Kolkata
https://www.shiksha.com/design/fashion-design/colleges/colleges-kolkata?uaf[]=credential&ts=5513598&rf=filters&ct[]=130&cr[]=11

There are lot of short management courses that you can pursue based on your graduation background and years of experience. NMIMS University offers a lot of similar courses.

IITs offer multiple short term courses and advertise their eligibilty criteria. Courses are open for all those candidates who meet eligiblity criteria defined by IITs. They need not be IIT students.

Hi Richa,
There are a few colleges to offer short-tem Diploma courses in education management and administration through distance education:
NMIMS
Symbiosis
IGNOU
IDOL- University of Mumbai
Amity University
You can choose them in the order of the ranks given above.
All the best.