Hi,
You should edit the master page, and locate "Quick Launch Menu" control and you should modify the StaticDisplay Levels & MaximumDynamicDisplayLevels, Something like this:
<SharePoint:AspMenu
id="QuickLaunchMenu"
DataSourceId="QuickLaunchSiteMap"
runat="server"
Orientation="Vertical"
StaticDisplayLevels="2"
ItemWrap="true"
MaximumDynamicDisplayLevels="1"
StaticSubMenuIndent="0"
SkipLinkText="">
Below post is a similar question:
http://social.technet.microsoft.com/forums/en-US/sharepointcustomizationprevious/thread/c07e9e8e-852d-412e-b844-a3939583c06d
Hope it might help you.
