when I take admission in b tech(mechanical engineering) . then I take admission another engineering college next year(computer science and engineering)
Dear aspirant,
You can change college after 1st year for engineering degree. But you will not be able to change your branch to CSE. If you want to do that than you'll have to give JEE mains again and start from the very 1st year itself. Many students have done that as well. So, that is your only choice if you really want to change college and branch you're pursuing.
Hope this helps!