i am giving improvemnt exam in 2020 if i will fill code 2 than in the place of 12th marks if i enter 12th class mark thn at the time of counsiling my mark will be chnged after imporvemnt exam result for that what should i do????

You shall fill the code as 02. Code 02 says: Higher/Senior Secondary Examination or the Indian School Certificate Examination which is equivalent to 10+2 Higher/Senior Secondary Examination after a period of 12 years study, the last two years of such study comprising of Physics, Chemistry, Biology/Bio-technology (which shall include practical tests in these subjects) and Mathematics or any other elective subject with English at a level not less than the core course for English as prescribed by the National Council of Educational Research and Training after introduction of the 10+2+3 educational structure as recommended by the National Committee on Education.

Now when you get the marks sheet for improvement exams it will be a different mark sheet and during the document verification, you can show both these marks lists. Ideally your 12th marks only are for checking the eligibility criteria for NEET but not to decide your merit. And so selecting 02 is fine.