Hello Riya

If you didn't want to take admission in the college allotted to you in the first round of counselling, donot take admission and fill the form for second round of counselling this way you drop the college of first round and then in second round college will be allotted to you and then if you wish you can take admission there. 

Once you loose the college allotted in first round you cannot take admission there , if you apply for second round of counselling. 

Any query you may ask further.