You need to add x:FieldModifier="public"
for the controls which you want to update in MemberWindow. Such as, put a lable named myLb in MemberWindow.xaml.
<Label Name="myLb" Width="400" Height="50" Content="This is MemberWindow!" FontSize="30" x:FieldModifier="public"></Label>
Then you can update its properties in other windows like below:
var MW = Application.Current.Windows.OfType<MemberWindow>().SingleOrDefault();
MW.myLb.Content = myTxt.Text.ToString();
MW.myLb.Background =Brushes.Red;
// you can also update the MemberWindow's field
//MW.field1 = "test string";
By the way, if I misunderstand your question, please point out and I will pay attention for your reply.
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