Hello Aspirant

The seat which you have been allotted is your final seat until or unless you get a better seat than it based on your rank and choice filling, so you can not delete the choice which has already been allotted to you.

But, you can do one thing, delete all of the choices of seats that you don't want except for the one which you have been allotted and the one which you want. Just keep the seat which you want on the first priority or first option. If you are lucky and have a rank good enough to get that seat then maybe you could get the seat you want.

Hope this helps