Здесь я сгруппировал кнопки.Интересно,есть ли ошибки в данном коде?))
counter = 0
GraphicsWindow.Width = 500
GraphicsWindow.Height = 300
GraphicsWindow.FontSize = 11
GraphicsWindow.Title = "Простейший текстовый редактор"
GraphicsWindow.BrushColor = "Blue"
Button_1 = Controls.AddButton("Открыть файл", 0,0) 
Controls.SetSize(Button_1, 300,50)

Button_2 = Controls.AddButton("сохранить файл",0,0)
Controls.SetSize(Button_2, 300,50)
Controls.HideControl(Button_2)

Button_3 = Controls.AddButton(">>>",250,0)
Controls.SetSize(Button_3, 300,50)

Controls.ButtonClicked = OnButtonClick
Controls.TextTyped = OnTextTyped

NewEdit_1 = Controls.AddMultiLineTextBox(0,50)
Controls.SetSize(NewEdit_1, 500,250)
NewEdit_2 = Controls.AddTextBox(0,200)
Controls.SetSize(NewEdit_2, 500,25)
Controls.SetTextBoxText(NewEdit_2,"filename.txt")
resulttext = ""
filename = "filename.txt"
Sub OnButtonClick
  if Controls.LastClickedButton = Button_1 Then
    result = File.ReadContents(filename)
    Controls.SetTextBoxText(NewEdit_1,result)
  EndIf
  
  if Controls.LastClickedButton = Button_2 Then
    File.WriteContents(filename,resulttext)
  EndIf
  
   if Controls.LastClickedButton = Button_3 And counter = 1 Then
    counter = counter + 1
    Controls.HideControl(Button_2)
    Controls.ShowControl(Button_1)
  endif
  
  if Controls.LastClickedButton = Button_3 And counter = 0 Then
    counter = counter + 1
    Controls.HideControl(Button_1)
    Controls.ShowControl(Button_2)
  endif
  
  
  If counter = 2 Then 
    counter = 0 
  endif
    
  
EndSub

Sub OnTextTyped
  edit = Controls.LastTypedTextBox
   If edit = NewEdit_1 then
   resulttext = Controls.GetTextBoxText(edit)
  EndIf
  If edit = NewEdit_2 then
   filename = Controls.GetTextBoxText(edit)
  EndIf

EndSub