Change binding/text of a label with a switch

You must place the event in toggled in your switch and when it is activated you change your text in the label and there you can correctly use the INotifyPropertyChanged interface.

I put some changes, first of all if your bindingContext in your label is your slider, the interface does not work for you because you make another binding instead of the property that you edited to carry out said work and the graphical interface never finds out about those changes.

Second, in your interface you had some badly written lines, you needed to set your value that you brought and in the event you passed a static text.

I added the event on the slider, when it changes the value, that value is passed to your weight property and this property notifies the graphical interface.

I made some changes so that everything works well, now you have to finish adapting it to your tastes and needs.

 public partial class BMICalculatorPage : ContentPage
{
    BMICalculatorPageViewModel bmi = new BMICalculatorPageViewModel();
    public Pagina3()
    {
        InitializeComponent();
        BindingContext = bmi;

    }

    private void Switch_Toggled(object sender, ToggledEventArgs e)
    {
        if (e.Value)
        {
            bmi.Weight =$"{Weight_Slider.Value * 2.205} lbs.";
        }

    }

    private void Weight_Slider_ValueChanged(object sender, ValueChangedEventArgs e)
    {
        bmi.Weight = e.NewValue.ToString();
    }
}
public class BMICalculatorPageViewModel : INotifyPropertyChanged
{
    string weight = string.Empty;

    public string Weight
    {
        get
        {
            return weight;
        }

        set
        {
            if (weight == value)
            {
                return;
            }
            weight = value;
            onActivateSwitch(nameof(Weight));
        }
    }


    public event PropertyChangedEventHandler PropertyChanged;

    void onActivateSwitch(string weight)
    {

        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(weight));
    }
}

     <Label  x:Name="Imperial_System_Label"
                Grid.Row="3"
                Grid.ColumnSpan="3"
                Text="Imperial System" 
                TextColor="Black"
                FontSize="22"
                HorizontalOptions="Center"/>

        <Switch x:Name="Switch"
                Grid.Row="4"
                Grid.ColumnSpan="3"
                HorizontalOptions="Center"
                Toggled="Switch_Toggled"/>

        <Label x:Name="Height_Label"
               Grid.Row="5" 
               Grid.ColumnSpan="3"
               BindingContext="{x:Reference Height_Slider}"
               Text="{Binding Value, StringFormat='Height: {0:F0} cm'}" 
               TextColor="Black"
               FontSize="22"
               HorizontalOptions="Center"/>

        <Slider Grid.Row="6" 
                Grid.ColumnSpan="3"
                x:Name="Height_Slider" 
                Maximum="240" 
                Minimum="135" 
                Value="170"/>

        <Label x:Name="Weight_Label"
               Grid.Row="7" 
               Grid.ColumnSpan="3"
               Text="{Binding Weight, StringFormat='Weight: {0:F0}'}" 
               TextColor="Black"
               FontSize="22"
               HorizontalOptions="Center"
               VerticalOptions="Center"/>

        <Slider Grid.Row="8" 
                Grid.ColumnSpan="3"
                x:Name="Weight_Slider" 
                Maximum="400" 
                Minimum="35" 
                Value="70" ValueChanged="Weight_Slider_ValueChanged"/>

It is important that you delve into the mvvm pattern since the documentation and examples will be in mvvm. In this case, since it is a toggle, it does not allow the use of commands, but the normal thing is that the events are handled by means of commands. This example might help you:

Modify the xaml as follows:

 <Grid>



     <Switch 
                    Grid.Row="4"
                    Grid.ColumnSpan="3"
                IsToggled="{Binding Toggled}"
                    HorizontalOptions="Center">

        </Switch>
        <Label 
                   Grid.Row="5" 
                   Grid.ColumnSpan="3"

                   Text="{Binding Height}" 
                   TextColor="Black"
                   FontSize="22"
                   HorizontalOptions="Center"/>

 <Label 
                   Grid.Row="7" 
                   Grid.ColumnSpan="3"

                   Text="{Binding Weight}" 
                   TextColor="Black"
                   FontSize="22"
                   HorizontalOptions="Center"
                   VerticalOptions="Center"/>

    </Grid>

Then create a ViewModel or modify the one you already made with this code:

public class vmMain:INotifyPropertyChanged
{
    private bool toggled;

    public vmMain()
    {
        Height = "70 cm";

        Weight = "70 cm";
    }



    private string height;

    private string weight;



    public string Height { get => height; set { height = value;NotifyPropertyChanged(nameof(Height)); } }
    public string Weight { get => weight; set { weight = value;NotifyPropertyChanged(nameof(Weight)); } }
    public bool Toggled { get => toggled; set { toggled = value;

            NotifyPropertyChanged(nameof(Toggled));

            handle();

        } }

    public event PropertyChangedEventHandler PropertyChanged;


    protected virtual void NotifyPropertyChanged(string propertyName)
    {

        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }


    private void handle()
    {
        if (Height.Contains("cm"))
        {
            Height = "155 lbs";

        }
        else
        {
            Height = "70 cm";
        }

        if (Weight.Contains("cm"))
        {
            Weight = "155 lbs";

        }
        else
        {
            Weight = "70 cm";
        }



    }


}

And finally, in the code behind the content page, modify it as follows:

public partial class BMICalculatorPage : ContentPage
{

    vmMain vm=new vmMain();// La instancia del VIEW MODEL

    public BMICalculatorPage ()
    {

        InitializeComponent();

        BindingContext = vm;
    }


}