After the declaration of first round, few days later second round will be conducted. Now if you are satisfied with the college and branch you got through the first round then you can freeze your choice by visiting any of the centres after round 1. But if unsatisfied then you have to visit the centre and ask for upliftment of your choice in the consequtive upcoming rounds. When the result of second round is declared , if satisfied then you can go to a centre to lock your college and branch that you got through second round. But again if unsatisfied then just wait till the third round.
Hope it helps 🙂