Ask for extra credit. Some teachers and professors are willing to assign extra credit work to students who seek a higher grade. While this isn't always an option, a "B" just in reach of an "A" can be raised with the additional points an extra credit project can provide -- effectively contributing to a higher GPA.
Enroll in extra classes for a couple of semesters. Instead of taking three classes, take four or five and simply plan to work harder. Good grades in more classes will significantly raise your GPA.
Hire a tutor to help you with subjects you struggle with. One on one help with difficult subjects helps with your understanding of the subject, which strengthens your focus and your confidence.
Go to class and study. These are obvious ways to improve your grades in two semesters, but they are the easiest and most effective method of improving your grades and your knowledge of any given subject. If your class has study groups -- participate. Learning with others often provides insight into a subject that may not have become clear through self study.