Question : write to app.config file

Can somebody tell me how to write to the app.config file.  I tried to do it with the code attached but can only write to the cached values. 
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Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        System.Configuration.ConfigurationManager.AppSettings.Set("ServerName", Server_Box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("DatabaseName", Database_box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("UserID", User_box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("Password", Password_box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("Payment_Ref", Payment_Box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("SMTP-Server", Smtp_box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("EmailFromName", Email_Name_box.Text)
        System.Configuration.ConfigurationManager.AppSettings.Set("EmailFromAddress", Email_Addr_box.Text)
        System.Configuration.ConfigurationManager.RefreshSection("AppSettings")
    End Sub

Answer : write to app.config file

This appears to work:

Cells(rn, 6) = Obj.TextFrame.Characters.Text
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