Sadly, after your first two years, it can be rather difficult to change or boost your GPA, especially if it was lower than you would have liked during your freshman and sophomore years. Even with good grades in the final years of high school, the damage may have been done and often, your senior year classes doesn't have a factor into the GPA the colleges see.
Your ACT or SAT score offers you another opportunity to show the college or university how far you've come academically, even if your GPA is a little lower than expect or want. It's your opportunity to show you can work under pressure, study hard, and perform well. This is a major reason why you should study and be prepared on test day.
Studying and taking the ACT or SAT exams can be an extremely daunting task. However, it's good to remember that they are extremely important to your future. Your colleges will look to them during the early application process to see if you're a strong fit at their school. If you feel you could improve upon the score after your first attempt, try to retake the exam after more practice. A high ACT or SAT score will pay you back!