Let me take a little of your time to clear this out, So in this case we are talking about Updating the Source not the Target right!
So mainly there are two methods we can use:
UpdateSource() Method updates the source property of a given binding with the current value in the target Control Property.
This method does nothing when the Mode of the binding is not TwoWay or OneWayToSource.
UpdateTarget() Method forces the target Control to updates it’s property value, from the source property.
This method is useful if your source property model doesn’t implement
INotifyPropertyChanged or any other property changed notification mechanism.
TextBox.TextProperty by re-pulling a fresh value from the source model:
To send the latest
TextBox.TextProperty value (in the UI) to the source model property:
UpdateTarget() is to update the User interface (The Control) while
Up dateSource() updates the bound model property.
Hope this helps explaining the difference, and how both can be used.