Easy enough. Home>settings>lessons>reset lessons. Don't think it can be done for just one group although I don't know for sure. You can redo any of the old lessons though. I often revisit some of the tactical motifs by GM Wolff.
Reset lessons

Should be one of the big red buttons. Hope this link works...
https://www.chess.com/settings/lessons
And it does. Don't feel bad; it's a confusing site.
Is there any way of resetting lessons? I have completed several lessons but I would like to reset one group of lessons so I can start them all over..
How can I do this?
Thanks