Hello dear, If you want to cancel your admission or withdraw your seat from a college firstly you need to check their official website and check there is any option for withdrawing of your seat. You directly withdraw your seat or else if you are unable to do it online you may go to respective admission cell of that college and apply for you withdrawal.

And after your withdrawal according to UGC act you will get your fees refund. And this amount will vary according to your time from your admission. You can check the official website.