how to access an XAML control from code

i got a textbox control that for some reason I cannot set its text propertie from code. I think because it is part of data template but not sure. is there a way to access it. here is the XAML code.

I need to change the UnitUsedTextBlock.Text . thank you

<ListView x:Name="RouteResultsListView" ItemsSource="{Binding RouteLegs}" Width="400" Height="540" HorizontalAlignment="Left"   SelectionMode="None" ScrollViewer.VerticalScrollBarVisibility="Disabled">
                    <ListView.ItemTemplate>
                                <DataTemplate >
                                    <ListBox  ItemsSource="{Binding ItineraryItems}" Height="499">
                                        <ListBox.ItemTemplate>
                                            <DataTemplate>
                                                <StackPanel Orientation="Horizontal"  >
                                                        <TextBlock  Text="{Binding Instruction.Text}" 
                                                               TextWrapping="Wrap" MaxLines="5" Width="200" />
                                                        <TextBlock Text="{Binding TravelDistance}" 
                                                               Margin="10,0,0,0" />
                                                        <TextBlock x:Name="UnitUsedTextBlock" Margin="5,0,0,0" Text="km" />
                                                </StackPanel>
                                            </DataTemplate>
                                        </ListBox.ItemTemplate>
                                    </ListBox>
                                </DataTemplate>
                            </ListView.ItemTemplate>
                        </ListView>