There are two scenarios:
I. The lesson was created by you or you have been granted edit rights
When you have edit rights, you see an unlocked icon, stating 'editable'.
In this case, select the challenge, make sure you are in edit mode, click 'more' and click 'Remove from subject' and confirm.
If the exercise was created by you or you have edit rights for it, you can still access the exercise from your repository.
If the exercise was created by someone else and you do not have edit rights, it will no longer be visible to you.
II. The lesson was not created by you and you have not been granted edit rights
In this case, you must first copy the lesson, because you cannot edit the lesson itself.
Once you have created your own copy of the lesson, go to the exercise(s) you want to remove and follow the steps explained above under section I.