Editing the main slider’s video/slides
Simply edit the page with Elementor and click on the slider itself. You should be able to see all options of the Main slide as well as the other ones.
A quick recording doing this can be found here https://d.pr/v/rO8xsL .
Customizing the header
The header is overridden in the homepage’s backend settings to position itself over the slider and overlap eg: https://d.pr/i/b1mlKj .