I have scored 140 in jee mains and 93.2% in CBSE class 12 exam. I couldn`t perform well in advance . I have a doubt that should I take a drop of a year and prepare again or take any branch in nit or iiit and try to change my subject next year or I should join any govt college with the branch of my choice .

It depends on your choice, whether you really want to get into an IIT. If you are more institute specific, its worth dropping a year, and preparing hard for JEE Advanced.

However, if you are more branch specific, you can take your branch of interest in any reputed government college.

Now regarding the branch changing thing, you should know that for changing branch after first year, you will have to work a lot harder than your batchmates, to be at the top of the class. Very few top rankers from different batch are given chance to change their branch.

So take your decision accordingly ! Good Luck !