First ask, "Do I know the strategies to optimize my score?" There are test-taking strategies, which will improve your performance. Example: Avoid spending too much time on a question that you can't answer quickly, move forward to the next! There are books and guidance out there about such tactics, you may want to scan through one or two for ideas.
Another consideration is test preparation. You can do this on your own, by brushing up on math problems, reading and writing, or you can pay for a service to help. Test prep on its own won't make you "smarter", but it will help you face the more challenging questions with better poise and confidence. There are on-line services, too; see https://www.chegg.com/test-prep. There are likely local services available in your community as well.
Taking the SATs is a stressful endeavor; however, there are many ways to prepare for the test.
The methods will vary by each individual, so try a few different methods and see what works best for you.
There are many courses offered by a third party that help prepare students for the SAT. The courses could be in person classes or online. I recommend looking into these classes, researching what each class has to offer, what are the areas of focus, when the class is offered, the length of the course, and cost. If you know a specific area that needs the most important, check out Khan Academy online. They have hundreds of thousands of videos on a variety of subjects that you can choose and tailor your studying.
The other option is purchase SAT preparation books. Once again, there are many companies that make/offer these books, and the books will sometime overlap with the in-person courses. I recommend researching these books to see if they will benefit your study style. The books will generally cost less than the classes.
After you find the method of study that works best for you, I recommend taking practices tests. There is no better way to implement and reinforce what you have studied by taking practice tests under real testing conditions (using the same time constraints). After reach practice test, go back and study areas that you missed questions in. The SAT is a tough test, but with strong study habits, you will do great!