Create a dispatcher timer and for every ticks update your textbock.
number = 0
dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
dispatcherTimer.Tick += new EventHandler(dispatcherTimer_Tick);
dispatcherTimer.Interval = new TimeSpan(0,0,1);
dispatcherTimer.Start();
private void dispatcherTimer_Tick(object sender, EventArgs e)
{
//if you use binding and mvvm
this.Text = number.tostring();
//if you don't use binding
yourTextblock.Text = number.toString();
number ++;
}