Your program end date on your Form I-20 must always reflect (to the best of your knowledge) the date you will finish your studies. If you do not think you will be able to complete your program by the completion date on your I-20, or if you are unsure (e.g., if your graduation is dependent on a borderline GPA), you must apply for a program extension before the program end date expires and get an updated I-20. It is not possible to extend your F-1 program after the I-20 end date has passed.
To apply for a program extension, you will need the Program Extension Form, which is available from the Forms and Documents page of the International Admissions and Student Services website. If you expect to complete your program earlier than the program completion date, you must get an updated I-20 before you complete your program.