Reviewing previous levels
I haven't been on in a while and when I stopped I was on level 6. How do I get back to level 1 to review?
The level indicates how many experience points (XP) you have received for time spent on DuoLingo. It doesn't actually reflect anything in regards to ability level of a language nor of degree of difficulty of content being given to you.
So you can just click on any and all of the lessons that you had previously completed and do them again! Hope that is clear and I hope that others chime-in if I missed something.