You haven't mentioned the entrance examination you are enquiring about. But if this is about NEET PG, then unfortunately you cannot change your Category during the correction window either. NBE has disallowed any corrections of basic information such as Name, Date of Birth, Nationality, Gender, Email Id, Mobile no#, Photograph, Signature, Exam City and Category. To avail OBC reservation, you may have to re-register using another Email Id and Mobile no# and fill in another application which means its altogether a new payment again. But you cannot change your Category.