If you want you can reject your seat or don't take admission in it and then reattempt the examination next year. Or you can float and see if there are any upgrades available for you in the next rounds. Its dependent on you and based on your interest you should take the decision.
To know more about the IAT counselling visit the link below:
https://engineering.careers360.com/articles/iiser-counselling
I hope this answer helps. All the very best for your future endeavors!
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