Yes, there is a way to reset a lesson back to the start.
You will need to go to the Mastery tab for the lesson, and at the bottom you should see a "Reset levels" option:
When you click on Reset levels, you will receive a confirmation warning of what you are about to do, telling how many entries (words or phrases) will be affected, and the change in score which will occur:
When you click Reset, all the entries for that lesson will be reset to the Unstarted state, and the points for those entries will be reset to zero:
Why would you want to do this? Well, if you feel you just need a little bit of extra practice for a particular lesson, then by resetting the lesson scores, Lingopolo will automatically quiz you again on this lesson content.