Girija, your 12th marks do not affect your merit in NEET. You are asked to enter your academic details to guage your qualification. But again because you have already one used the correction facility once, there is  nothing that can be done about it. For now what we would suggest is to let the application be the way it is and during document verification eitherways as you will submit the original and xerox copies of your boards exams, you will be fine. Do not worry about it and concentrate on the preparation.