Horizontal layout is default layout in some of theme types but if you want to change it then by changing data-layout="horizontal" from other layout style the verticle layout can be set.

data-layout="horizontal"

How to add new items in menu or make changes in menu items?

In order to add, change or remove menu items from the left side navigation, simply edit in file src/layout/LayoutMenuData.tsx. The change would reflect in all the html files automatically.