I am using MahApp MetroWindow which has a title template like below.
<metro:MetroWindow x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:metro="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
xmlns:local="clr-namespace:WpfApplication1">
<metro:MetroWindow.TitleTemplate>
<DataTemplate>
<Grid>
<TextBlock Text="New Title"/>
</Grid>
</DataTemplate>
</metro:MetroWindow.TitleTemplate>
Add Title property to window and do templatebinding on the datatemplate. Try this.
<metro:MetroWindow x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:metro="clr-namespace:MahApps.Metro.Controls;assembly=MahApps.Metro"
xmlns:local="clr-namespace:WpfApplication1"
Title="New Title">
<metro:MetroWindow.TitleTemplate>
<DataTemplate>
<Grid>
<TextBlock Text="{TemplateBinding Content}"/>
</Grid>
</DataTemplate>
</metro:MetroWindow.TitleTemplate>