You are able to repeat any course or lesson as often as you like.
To repeat a lesson, use the progress bar in the active course window on your Babbel homepage: the orange sections indicate lessons already accessed. Click on the relevant section and then click Start.
To repeat an already completed course, select the course from Courses at the top of your Babbel homepage and then work through the individual lessons.
In the app, just select the course you want from Courses in the app's menu and select the lessons you want to repeat.