From the College's Course Catalog:
Letter grades on file with the Registrar at the end of a semester are final unless an error in calculating the grade is discovered. If so, the instructor must file a grade change form, explaining the error. This form requires the approval of the department chair and is then processed by the Registrar.

You would speak with the instructor regarding the error. If your instructor is not available, I would suggest calling the Registrar's office for further instructions.