Hi! Yes, you can do your M. Tech in computer science after B. Tech in CSBS. At the same time,  it might be very challenging because you will have to take GATE in CS, therefore have a good knowledge about it and score exceptionally well to get into top institutes like IITs and NITs. Basically, you can switch your branch in MTech if you are interested, but you will have to be mindful of its course as well. Hope this helps! All the best!