The only way you can change your grade is by doing a retake exam. Your college has refused you this option, this happened to me too. I'd look into other colleges who might let you resit the exam.
The alternative is trying to get into another uni with the grades you got. I know how you feel, I'd set my heart on one uni but couldn't go there because I had dropped a grade. Ended up at a total other uni in a totally different part of the country. Still I passed with a good grade and met good friends there
