While I recommend you start from the beginning and earn your way through the lessons, yes you can skip ahead if you’d like:

Mac/PC
  1. Open the Table Of Contents by clicking the “<<" button at the top left
  2. Right or CTRL click any locked lesson and select Unlock Lesson
  3. Click the Go button
iPad
  1. Tap the Menu button at the top left and select “Table Of Contents”
  2. Swipe left or right on any locked lesson to unlock
  3. Tap the Go button