Hello,
refer to this video:
https://youtu.be/C0RQO3-tjAI
this is for the setup wizard, you have to follow the same steps as shown in the above link.
yes, you can delete the demo contents. those are just dummy so that user can get an idea that how to create a course in a proper manner. you could simply put them into the draft or goto
wp-admin >> lms >> all courses/questions/units/quizzes
select the items and click action bulk delete.