How to change which window opens first in C# WPF?

It is rare we need to change the startup opening windows in WPF but if you need to change the startup window

Open App.xaml file, it will look something like below, where we need to update StartupUri

<Application x:Class="WpfHacking.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml"> <!-- This is the line we need to update -->
</Application>