WPF change UserControl textblock from another window

Question

Hey all I am new at WPF so here is my question:

How can I change the text in a TextBlock from my mainWindow when the textBlock is in the window named curTemp.xaml?

curTemp.xaml code:

public partial class curTemp : UserControl
    {
        public string _valTempChange
        {
            get { return middleForcastCurrentTemp.Text; }
            set { middleForcastCurrentTemp.Text = value; }
        }

        public curTemp()
        {
            InitializeComponent();
        }
    }

Xaml of the above UserControl:

<Grid>
        <TextBlock TextWrapping="Wrap" Padding="5" Foreground="White" Panel.ZIndex="7" HorizontalAlignment="Left" VerticalAlignment="Top" Margin="15,-110,-43,0" Width="198" Height="122">
            <TextBlock.Effect>
                <DropShadowEffect BlurRadius="4" Direction="0" ShadowDepth="0" />
            </TextBlock.Effect>
                    <outlineText:OutlinedTextBlock Height="146" Width="192" TextOptions.TextRenderingMode="Aliased" FontFamily="Segoe UI" FontSize="100" x:FieldModifier="public" x:Name="middleForcastCurrentTemp"
                                            FontWeight="Medium" TextWrapping="Wrap" TextAlignment="Right" Stroke="Black" StrokeThickness="3" Fill="White" Text="10"/>
        </TextBlock>
    </Grid>

 And in my MainWindow code:

public MainWindow()
        {
            InitializeComponent();

            curTemp _curTempWindow = new curTemp();

            _curTempWindow._valTempChange = "55";

        }

When I run that code it never shows "55" in the textBlock. It only shows my default text "10".

What am I doing incorrect here?