You are able to repeat any course or lesson as often as you like.
To repeat a lesson, use the navigation arrows in the active course window on your Babbel homepage to move between lessons. Click Start when you've found the one you want to work on.
To repeat an already completed course, select the course from either My Level or More Courses at the top of your Babbel homepage and then work through the individual lessons.
With an Android or iOS app, just select the course you want from Courses in the app's menu and select the lessons you want to repeat.