Very easy. Click on the Classroom at the top. Be sure to click on "Show Completed Items" on the left and it will show you all the items you've completed before. By doing so, you'll be able to visit previous lessons you've completed.
A simple way to view recent lessons is to go to your activity on the home page. There should be something like, "You read the lesson____!" Click the lesson name and you'll be back. However this only works if you've done it recently; to view older lessons you have to go to the Classroom.