Check to see if you can retake the class. Some schools won't let you retake classes in certain situations, such as if you received an "Incomplete" the first time. Others will let you take any class as many times you want, no matter what you received the first time. If you have completed another course in which the course you want to retake was a prerequisite, the retaken grade may not be able to be factored into your GPA.
Register and pay for the class that you want to retake. Some schools will not count a retaken class toward your workload for the semester. In this case you would need to take the extra class on top of your regular 12 credit hours to be considered full-time. Also, the class will only count towards your total hours needed for a degree once. Make sure you retake the class at your current school. When you transfer credits the grades do not transfer with them, which means the retaken class won't help your GPA.
Study hard and get a better grade. There is little point in retaking a class if you are going to get an equal or worse grade than you did the first time, and it some cases it may even hurt you. You will need to check with your school to see which grade will be factored into your GPA. Some schools take the higher of the two grades, while other schools take the last grade attained, even if it is worse than the first grade. You will also need to see if both grades will be kept on record or if the retaken grade will replace the original grade on your transcript.