Just click on Teaching, then Courses, and click on the course that you want to edit. Once in the course, ensure that you are in the General Settings tab.
Next, scroll down until you see the Course Certificate section. In the Validity Length field, simply enter the number of days that you want the course to be valid for after a learner completes it. Once the time has elapsed, the learner will automatically be enrolled again. For example, if you needed a safety course done every year, you could set the Validity Length to 365, ensuring that all of your learners will be re-enrolled once the year is up.