Hello Shivam

You have to give the preference list first where you have to specify or list out the colleges which you want to get in most preferred to least preferred sequence. If you get some college in 1st round then in subsequent rounds you will get the college which are above in the list. If you get another college that means you are getting more preferred college and you can't take the previous option. Other happening  can be you won't get any new college and remain on the previous one only.

Hope this helps.