Its okay if you opt for hindi in u.p board you should give exam in hindi as u.p board has the textbooks for your class 12 th are printed in Hindi language. And secondly, the question paper mentions all the questions in Hindi and You will have to write answers in the language which you chose earlier filling up the board exams's form. It was filled by your class teacher you should follow that otherwise you may face consequences later.many students say that they are changing from Hindi to English medium in exam and there is no issue in results but you never know the same happened to you too.if you still think you should give it in english contact your class teacher.

BEST OF LUCK...