Switch Navigation in TreeView WPF

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C#

WPF

C#4.0

I have a control populated with data like this

Label1
---Field1
---Field2
---Field3
Label2
---Field4
---Field5
---Field6

i have two buttons Next and Back. Now Field1 is selected on clicking Next i have to show Field2 as current selected item. After Field3 if i click Next current selected item should be Field4. vice versa when i click Back.

My treeview structure is like this

<TreeView x:Name="FieldTreeView" Grid.Row="1" BorderBrush="White" BorderThickness="0" ItemsSource="{Binding TreeDataSource}">
                            <TreeView.Resources>
                                <HierarchicalDataTemplate DataType="{x:Type self:Labels}" ItemsSource="{Binding FieldName}" >
                                    <TextBlock FontWeight="Bold" Text="{Binding Name}"/>
                                </HierarchicalDataTemplate>
                                <DataTemplate DataType="{x:Type self:Fields}">
                                    <TextBlock Text="{Binding Field}" />
                                </DataTemplate>
                            </TreeView.Resources>
                            <TreeView.ItemContainerStyle>
                                <Style TargetType="{x:Type TreeViewItem}">
                                    <Setter Property="IsExpanded" Value="True"/>
                                </Style>
                            </TreeView.ItemContainerStyle>
                        </TreeView>

My class structure

public class Labels
{
    public Labels()
    {
        this.FieldName = new List<Fields>();
    }
    public string Name { get; set; }
    public List<Fields> FieldName { get; set; }
}
public class Fields
{
   public string Field { get; set; }
}

Populating my data like this

List<Fields> fld = new List<Fields>();
            fld.Add(new Fields() { Field = "Field1" });
            fld.Add(new Fields() { Field = "Field2" });
            fld.Add(new Fields() { Field = "Field3" });

            List<Fields> fld1 = new List<Fields>();
            fld1.Add(new Fields() { Field = "Field4" });
      




      fld1.Add(new Fields() { Field = "Field5" });
            fld1.Add(new Fields() { Field = "Field6" });

            TreeDataSource.Add(new Labels() { Name = "Label1", FieldName = fld });
            TreeDataSource.Add(new Labels() { Name = "Label2", FieldName = fld1 });

Can any one please help me on this.