can anyone change his branch after 1st year exam in bit sindri by doing good performance in exam,example from production to mechanical etc,o

It's totally depends upon the college because 1st year of engineering will be same for all branches and from 2nd year, there respective core subjects will start. So, if any seats are not filled in mechanical, you can opt that by considering management. Its only depends upon the management but this will affect some benefits like fees reimbursement etc..,
All the best.