How do I change the nav portlet?

What I want to do: Change the navigation portlet so that only the current top folder and its contents appear.

The fix:

Go to the ZMI in Plone.
Go to portal_properties → navtree_properties
Uncheck “includeTop”
Put the “topLevel” equal to 1.

Source: Plone Users mailing list archives

Someone also posted a howto in the Plone forums: