Mark lessons as unused

Sadly I got out of the habit of doing my lessons so now I need to back up a little. I'd like to mark some of the lessons I've already done (some considerable time ago) as unused so I can keep my place and re-do them in a logical sequence. Any ideas on how I can do this please?

Hi Brian - We used to have a reset button under the My Level tab. With the new version we did away with that. We will look at reinstating it in the next few weeks.