Hi Ravi,

  You can change the order of elements in the Navigation menu by dragging the elements above and below one another.  This allows you to set the order of the elements.  You can also use the 'New Subitem' to create nested trees of navigation.  Can you be more specific about what it is you would like to accomplish?  Here is a link to the navigation documentation:

https://docs.mendix.com/refguide/navigation