Theme Settings tab click the first one called
header , now you will find some text boxes with title. You can easily understand where the text box is used for, with the help of title.
After creating a navigation from your dashboard you can just select this menu for main navigation. Then you will find a checkbox for mega menu. if you want a mega menu just check this box. and then you can select your mega menu just like you select the main menu. after that you can choose an image for your mega menu. and you can choose your store