I have completed Lesson twice and It says that I can go to Lesson 17, but it keeps coming up as Lesson 16. How can I change this?
Levels and lessons are not the same. To reach the next level you simply have to gain more points by reviewing lessons.
However, if you have completed all the lessons (topics) on the homepage, and a golden owl statuette has appeared at the bottom, then you have completed the course.
You can continue to review, or when you feel you have nothing else to learn from the material, you can move on to other resources.