Example of MonoGame Update method updating ViewModel data

UpdateXaml.cs

protected override void Update(GameTime gameTime)  
{
    // add points if a projectile is out of screen
    foreach (var activeProjectile in activeProjectiles)
    {
        if (activeProjectile.Position.Y > ScreenHeight)
        {
            ScoreViewModel.Instance.BallPoints++;
            activeProjectile.Deactivate();
        }
    }

    base.Update(gameTime);
}