The mark that shows on your grade report for this course as of now, i.e., an F, is what you have right now out of the total hundred percent since the system calculates it like that. I am guessing your exam would be worth quite a lot which is why whatever you've scored in this course must be less than a 50 and so it shows as an F. Say if you had already completed evaluations worth about 75% of the total available grades in the course, you could be having a C or something like that on your grade report. So, just wait it out and it should change after you write the exam