Find the .css file usually its style.css
wp-content/themes/yourtheme/style.css
Normally when menu items are removed (from the menus admin screen or the customizer), what is displayed adjusts automatically. It’s strange there are empty elements. You’re best off asking for advice through your theme’s dedicated support forum.
Going from drop down style to tabbed style is a major change in CSS, but it is possible if the menu HTML is well formed, usually as ul/li elements. You could search for tabbed menu CSS examples and adapt them to work with your menu’s classes. You’ll need some reasonable CSS coding skills.