1) Better to salvage your GPA and take the PI received D in my first biochem class and I just retook it and got a C. I still have a chance to change my grade to pass and fail. Should I change Biochem to P/F even if I retake to again later on because having a C brings my gpa down significantly?
2) If you're only getting a C the second time around, something is seriously wrong with your learning style.
3) As such, you need to fix this before anything else. Go visit your school's learning or education center.