To 'Add/Remove Sub-Menu Items', from the store dashboard go to:
> Appearance > Menus

From within this page at the top, click the drop-down box next to where it says 'Select a menu to edit' and find the menu you'd like to edit, then click 'Select' to edit a specific menu on your store:

To create a 'sub-menu item', simply drag and drop one of the 'menu items' underneath and to the right of the parent. For example, if I wanted 'Back to Home' to be a 'sub-menu item' of 'About Us', this is what it would look like:

You can also create 'sub-menu items' for 'sub-menu items'. For example, if I wanted 'Delivery & Returns' to fall under 'Back to Home' (which is a 'sub-menu item' of 'About Us'), it would look like this:

To remove a 'sub-menu item' simply drag to the left. For example if I wanted 'Delivery & Returns' to be a 'sub-item' of 'About Us', but not 'Back to Home', it would look like this:

(*Once you're satisfied with the arrangement, don't forget to click 'Save Menu' at the top right*)