If you get a grade on Edexcel GCSE A-level exams, can you change your grade by reappearing for just one unit?

I recently gave my A-level exams. Results came out today and I managed to get A's in all my exams except math. My dad ped away and due to that, I had a horrible C4 exam. I already got B as a grade. If I retake C4 again, and do better, will my grade change? And if so, how do I retake it? Please help me out here. =)