SPSite site = new SPSite("sitename");
SPWeb Web = site.OpenWeb();
SPNavigationNodeCollection nodes = Web.Navigation.QuickLaunch;

SPNavigationNode NavNodeWorkingDoc = null;
SPNavigationNode NavNodePicLib = null;
SPNavigationNode NavNodeRefLib = null;
SPNavigationNode navNodeFinalDoc = null;
SPNavigationNode navNodePublished = null;
SPNavigationNode navNodeDisclosed = null;

foreach (SPNavigationNode node in nodes)
{
if (node.Title.Equals("Documents")) //Documents
{
foreach (SPNavigationNode childnode in node.Children)
{

if (childnode.Title.Equals("Documents"))
{
childnode.Delete();
//childnode.Url = "url name"
}
else if (childnode.Title.Equals("Project Documents"))
{
NavNodeWorkingDoc = childnode;
NavNodeWorkingDoc.Title = "Working Documents";
NavNodeWorkingDoc.Update();
}
else if (childnode.Title.Equals("Picture Library"))
{
NavNodePicLib = childnode;
NavNodePicLib.Title = "Picture Gallery";
NavNodePicLib.Update();
}
else if (childnode.Title.Equals("Reference Library"))
{
NavNodeRefLib = childnode;
}
}

node.Children.Add(navNodeFinalDoc, NavNodeRefLib);
node.Children.Add(navNodePublished, navNodeFinalDoc);
node.Children.Add(navNodeDisclosed, navNodePublished);
}
}