You can retake the course which received D and lower. After you retake the course, the previous grade that you received at D or lower will not be calculated in your GPA. However, both courses' grade will appear on the transcript. Also some of the college does not replace the grade automatically, you have to do a request through counselor before they take out the bad grade from GPA calculation. Also note, I heard from the recent news, due to the California Budget Cuts, community colleges no longer allow students to retake a course they failed starting this fall, but please double check with your school.