You can access your Classroom once you log into your account or directly from learn.udacity.com/. On the left hand side of your Classroom is your navigation menu. Here you will find the navigation links to access your help services, as well as your account settings.
If you're enrolled in multiple Nanodegree programs, you'll see all of them listed on your Classroom homepage. From there, you can choose which program to work on and also access any Nanodegree programs you've graduated from.
Once inside a specific Nanodegree program, you can navigate through the lesson using the menu on the left side of your screen. If the menu is hidden, click the three horizontal bars in the top left corner to reveal it. To switch to a different lesson, click the lesson title at the top of the navigation menu. This will take you to the Syllabus view, where you can navigate to any part of your course or program.