1. Manually editing the student and adding course to the student.
2. Use Course Updation Tool
You can add courses of next academic year to multiple students at once, with the help of Course Updation tool. You can access this option under Administration menu.
Course updation tool lets you add courses to multiple students by simply selecting the academic year and course along with the required set of students. You can also make use of filters to get the exact list of students that you need to the course add for.
P.S. In case, you have added the course to the student during enrollment, you should not add it again for such students.