I got a seat in nit through JoSSA but not satisfied with branch. Can i apply CSAB for better branch?

If you got a seat and you have not locked the branch, then wait for the subsequent rounds, as you might get a better seat in the next rounds. If you are still not satisfied even after the last round, then wait for the spot round, they will give you the desired seat if it is empty. If you have locked your seat, then there is no chance for upgradation. However, you can start your college with the branch you are currently in and then apply for branch upgradation next year. All NITs allow branch upgradation on conditions that atleast one seat should be empty in the branch you are lookin for and your CGPA should be above average. If these conditions are satisfied, then you might get a better branch before starting the second year. But, right now, you can not get a better branch unless you have a better JEE rank.