how to change a textblock value outside a usercontrol if the textblock is in another window

Question

My main goal here is to change the sidebar text after the user UPDATES the username.

My sidebar text (the one that displays the username) is housed in my MainWindow.xaml:

<Window //namespaces and properties here  
        WindowStyle="None"
        AllowsTransparency="True"
        Background="Transparent">

    <Border Background="#222529"
            CornerRadius="20">
        <Grid>
        
            <!--Removed some UI elements for simplicity-->

            <!--I have a sidebar menu here which I can navigate-->

            <!--Username (The text that I want to change after updating User's username-->
            <TextBlock Text="{Binding CurrentUserAccount.Username}" />

            <!--Where the views are going to be displayed-->
            <ContentControl Content="{Binding CurrentView}" />
            
        </Grid>
    </Border>
</Window>

This is my UserAccountView.xaml, and I update User details here

<UserControl //namespaces and properties
             Background="Transparent">
    <Grid HorizontalAlignment="Center">
             <!--Removed some UI elements for simplicity-->
             
            <!--Username Textbox-->
            <TextBox Style="{StaticResource TextBoxTheme}"
                     Text="{Binding CurrentUserAccount.Username}"/>

           
            <!--Password Textbox-->
            <usercontrols:BindablePasswordBox Style="{StaticResource BindablePasswordBoxTheme}"
                                              Password="{Binding CurrentUserAccount.Password}" />

            <!--Update Profile Button-->
            <Button Content="Update Profile"
                    Style="{StaticResource GeneralButtonTheme}"
                    Command="{Binding UpdateCommand}" />

            <!--Check current user details (just a temporary button to check user details)-->
            <Button Content="Check details"
                    Style="{StaticResource GeneralButtonTheme}"
                    Command="{Binding CheckDetailsCommand}" />
        </Grid>
    </Grid>
</UserControl>

I know that the User's details are Updated because of the temporary button that can check the user's details (and of course, the user details change when I login again). This is how I implement the check:

MessageBox.Show($"Id: {CurrentUserAccount.Id} 
                \nUsername: {CurrentUserAccount.Username} 
                \nPassword: {CurrentUserAccount.Password} 
                \nDate Created: {CurrentUserAccount.DateCreated}");

This is how I navigate through my menu (if this is relevant to the question)

class MainWindowViewModel : BaseViewModel
{
    public ICommand DashboardViewCommand { get; }
    public ICommand ProfileViewCommand { get; }
   // and other commands for each View
   
    public DashboardViewModel DashboardVM { get; set; }
    public UserAccountViewModel ProfileVM { get; set; }
   // and other ViewModels 

    public object CurrentView { get; set; }
    public UserAccount CurrentUserAccount { get; set; }

    public MainWindowViewModel(UserAccount currentUserAccount)
    {
        CurrentUserAccount = currentUserAccount;

        DashboardVM = new DashboardViewModel();
        ProfileVM = new UserAccountViewModel(currentUserAccount.Username);

        CurrentView = DashboardVM;

        DashboardViewCommand = new NavigationCommand(o => { CurrentView = DashboardVM; });
        ProfileViewCommand = new NavigationCommand(o => { CurrentView = ProfileVM; });
    }
}

The DataTemplate of my Views (in App.xaml):

<Application.Resources>
    <ResourceDictionary>

        <!--Binding the View to its ViewModel-->

        <!--Dashboard-->
        <DataTemplate DataType="{x:Type viewModel:DashboardViewModel}">
            <view:DashboardView />
        </DataTemplate>

        <!--User Account (if I share ViewModels and remove this, then I can't see my View now)-->
        <DataTemplate DataType="{x:Type viewModel:UserAccountViewModel}">
            <view:UserAccountView />
        </DataTemplate>

    </ResourceDictionary>
</Application.Resources>

How is it possible to change the value of an element from a Window if I am "changing" if from a ViewModel (that is is a ContentControl)?

I have to ask this question since I have found nothing similar about this problem, and I don't really know what "Keywords" I should look up to, since this is my first time creating a WPF application.

EDIT: I already solved this problem thanks to @Clemens' comment (which gave me an idea on what to search), I was able to share ViewModels for my main page and my UserAccountView by removing the DataContext for my UserAccountView and set the DataContext at the UserControl level:

<!--this is what my UserAccountView looks like-->
<UserControl <!--properties and namespaces-->
             DataContext="{Binding Path=DataContext, 
          RelativeSource={RelativeSource AncestorType={x:Type Window}}}">

ANOTHER EDIT : I found another way of doing this by separating the MainWindowViewModel and UserAccountViewModel so that the MainWindowViewModel (where my UserAccount related code stays, because of having a "shared" ViewModel) will not get messy.

This is what my MainWindowViewModel looks like now

class MainWindowViewModel : BaseViewModel
{
    public ICommand DashboardViewCommand { get; }
    public ICommand ProfileViewCommand { get; }
   
    public DashboardViewModel DashboardVM { get; set; }
    public UserAccountViewModel ProfileVM { get; set; }

    public object CurrentView { get; set; }

    public MainWindowViewModel(UserAccount currentUserAccount)
    {
        DashboardVM = new DashboardViewModel();

        //this is the change (the UserAccountViewModel now accepts an instance of the useraccount, 
        //instead of the user's name, so when I change a property of that instance, 
        //it will be reflected to the binding text of the sidebar as well)
        ProfileVM = new UserAccountViewModel(currentUserAccount);

        CurrentView = DashboardVM;

        DashboardViewCommand = new NavigationCommand(o => { CurrentView = DashboardVM; });
        ProfileViewCommand = new NavigationCommand(o => { CurrentView = ProfileVM; });
    }
}

Now all code related about UserAccount will all be done in UserAccountViewModel instead of in the MainWindowViewModel.