Select a Dashboard and click the "Manage" button in the top right corner. Click "clone" in the dropdown menu and give the new dashboard a name and refresh interval. You will get a choice to go to the new dashboard, or stay in the one you made the copy from.
The new dashboard will appear in the bottom of the "Public dashboards" or "Private dashboards" list, based on what you selected/what you had the permission to select when you made the copy.
Select a Dashboard and click the "Manage" button in the top right corner. Then click the "Delete" button. This action can not be undone.