There is Nothing to do with your class 12th Marks while taking admission to IITs. IITs care about your class 12th Marks and that to minimal, there is no weightage to class 12th Marks in admission to IIT for graduation. You just need to cross 75% barrier to getting into IITs.

Admission into IITs is solely based upon your Ranks in JEE Advance which is Organized By IITs. JEE Advance score is used to calculate rank and admission is offered based upon the rank given that student meets the minimum 75% mark in class 12th.