I have alloted seat in second extended round and I paid the fee how can I cancel my seat? please help me

If you want to cancel your admission, you need to visit the college. As you said you even paid the fees. Visit the college and let the administration faculty know about this. Most of the Colleges return the fee where few of the college's return 50 to 80% of fees. So They will let you know about the fee refund and will cancel your admission. So visit the college as soon as you can so that you will have chances to get admission in another colleges