I highly recommend using a diagnostic tool that will help you find the areas that you need to improve, while not wasting your time on the stuff you know.
For my GED students, I always use the current Kaplan GED study guide, available in any bookstore or on Amazon for only about $25. It has diagnostic tests for each subject. After you take the test, you find out which areas are weak, then concentrate on studying just those areas.
If you need a GED tutor, contact me! ;-)
Note: I am not a third-party affiliate for Kaplan or Amazon. I just really like the tool and find it highly affordable and effective.