If I update my item , so the buyers who buy this item before I made update can be download updated item?
Yes, they can download future versions
Clients can re-download the item from their downloads page if they see the author has done any announcements on the item page.
Ask them to follow your twitter account, place where you announce updates.
If you have WordPress themes, use this: http://themeforest.net/forums/thread/simple-theme-update-class-using-envato-api/73278?page=1#636326