How to delete a header and title in the lessons

Short answer: you can't.

Long answer: you can't, in Rise itself. Once a course is published you could manually edit its CSS to hide the header element, but this isn't a change that's fed back to Rise so you'll need to do it every time you republish. Also, obviously Articulate would advise you not to do this; I don't think the example I give below will affect functionality/accessibility, but it's not like I've rigorously tested it.

So, the example:

1. Download your published course.
2. Open the zip and navigate to scormcontent > lib > main.bundle.css
3. Within your text editor, search for .page__header-container and add display:none; inside the curly brackets following the search term.
4. Save main.bundle.

That's it. If you try to open the course in a browser you should see that lessons no longer have their headers. The element I targeted removes the 'Lesson X of Y', lesson title and inscrutable horizontal line but retains the menu button, so learners can still collapse/expand the menu.