Hello Sovan.
You can definitely do a Mtech in cs after Btech in civil Without even taking a two year drop.You can simply give the GATE exam and based on your score ,you can look for your colleges.I would suggest ,if you have genuine interest in cs ,then only do Mtech with cs otherwise do it in civil cause it will totally change your domain.You can also take a year drop,prepare for GATE and also develop some coding skills.Taking drop totally depends on your preparation.If you can crack GATE with good score without doubt,there is no need for drop otherwise you might .Hope this helps.
Good Luck!
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