Hello!

The day on which the result is declared,then you have to login from the site and fill you choice there is no limit of choice filling .

The most common mistakes are done by student that they fill the option first which he get,never fill them first.for example if you have to choice 1.Cse in Nit kurukshetra (which you don't get according to last year.  2. Ece of Nit kuruksherta (which you get on last year rank) so in choice filling fill 1 as 1 and 2 as 2,so if in first round you get choice 2 then there will be chance of getting 1 in further round .

fill the option first which you what but can't get last year and fill according to your interest.last year opening and closing rank on below link .

https://engineering.careers360.com/articles/jee-main-cutoff-for-top-nits

All The Best!