I have already paid my admission fees in Assam engineering college but I would like to withdraw my seat.

Withdrawing your seat after the payment of the tution fees would cause you to forefait your tutuion fees amount and you will not be able to get further seats in the institutes as you will not be able to go for the further rounds. Now if you have no plan for the study further then you can cancel your admiasion in the allotted institute by going there personally. Withdrawing the seat would cause you unnecessary trouble and document work so I would suggest you not to withdraw and continue with the study if you have no critical reason.