A syllabus ensures a mutual understanding between the instructor and students. It establishes a level of certainty regarding course goals, organization, scope, and policies. In addition, a syllabus establishes a clear understanding of acceptable behavior, expectations on the level of student effort, and procedures for communicating with the instructor.
As the instructor, you should be very familiar with the content of the syllabus. This is the roadmap for the course and a contract of sorts. It is also the document that you should refer students back to when they have questions about the course.
Given you are inheriting an existing course, your course syllabus will already be complete. You will simply need to enter your information at the top (e.g., your office hours and ways students can reach you). Any other desired changes to the content of the syllabus should be discussed with your Program Director and Instructional Designer, as the syllabus reflects the existing course material.