You will raise it more quickly if you retake the classes that you got C's in. If you think you GPA is high enough to around the GPA cuts (not sure of exact numbers here, but I would think if you are 3.4-3.5+ you would be fine.) If they look more closely at your application after that and see upper level classes or master's work it would help then. If it on the lower side, even if you do take some upper level classes or a work on a masters, they might cut you before they look too closely and see those other courses.