how to access an XAML control from code

I would use data binding to bind the Text property to a StaticResource that can be changed in the code behind.

1. Add a class with INotifyProperty change interface to act as the container for Units (This class could be useful for other places in your application as well.)

    public class ViewModel : INotifyPropertyChanged
    {
        private string units;
        public string Units
        {
            get { return units; }
            set
            {
                units = value;
                RaisePropertyChanged();
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void RaisePropertyChanged([CallerMemberName] string caller = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(caller));
            }
        }
    }

2. Instantiate this class as a StaticResource in Page.Resources

    <Page.Resources>
        <local:ViewModel x:Key="myVM" />
    </Page.Resources>

3. Add the data binding to your TextBlock in the DataTemplate

<TextBlock Text="{Binding Units, Source={StaticResource myVM}}" Margin="5,0,0,0" />

4. In the code behind, access the Units like this:

            var vm = this.Resources["myVM"] as ViewModel;
            vm.Units = "km";

When you change vm.Units, the display will also change.