Hello,
In order to edit the main menu, please navigate to Appearance > Menu > choose that one, which assigned to the main menu. http://prntscr.com/jgfeve
As you can see some of the menu items have the drop-down mega menu. Enter a menu item and you will see the mega menu block http://prntscr.com/jgff47
Next, remove or add another HTML block or navigate to HTML Blocks, find it and edit http://prntscr.com/jgffis Entering an HTML block you will see WP Bakery Template
Best Regards