If you do not want to join the allotted institute in the first round then you need to go for the second round of the counselling and also you do not need to report to the institute for the confirmation. In doing so you can lose your seat in the previous round and as per the preferences you have filled in the second round of the counselling you will be allotted the institute based on the number of vacant seats. Also in the subsequent rounds of the admission, fill your choices very carefully as the number of seats always less in the sccessive counselling rounds.