Default it will have a Vertical layout. It is a classic and clean design for a traditional admin experience. Do the below changes in the Admin/src/partials/main.html file. In the html tag add the attributes or update the value of existing attribute. Make sure to clear cache or open in incognito mode to achieve the changes.
data-layout="vertical"
data-content-width="default"
data-content-width="box"
data-sidebar="default"
data-sidebar="icon"
data-sidebar-color="light"
data-sidebar-color="dark"
