How to add sub-categories to navigation ?

Hi,

Can any one tell me how can i add sub-categories of a category in Navigation tab to display them once they click on specific category ?

I am using Arras theme.

For ex: If i have categories like A, B, C and D

A has sub-categories -> E, F, G
B has sub-categories -> H, I, J

How can keep sub-categories in the navigation so that if user roll his mouse on A, it has to show its sub-categories too ………