The Free Application for Federal Student Aid (FAFSA) collects data as of the date you complete the form. This is used in calculating your financial aid eligibility for the coming year. Only limited changes in your household situation are allowed to be updated after filing your FAFSA.
Marital status is not one which can be updated. Marital status is set for the year, for aid purposes, as of the date you complete your aid application. It cannot be answered with a prediction or updated afterwards.
If you are planning on getting married, you should consider whether waiting to complete your FAFSA form until after your special day would be beneficial to your aid calculation. Marriage can affect dependency status, household size, number in college and adjusted gross incomes. All of these are major factors in the financial aid calculation.
Feel free to speak with your financial aid counselor early on about estimating which timing of filing your FAFSA may be best for your situation.