this worked for me:
Imports Microsoft.Office.Interop.Excel
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim excelApplication As New Application
Dim excelWrkBook As Workbook
Try
excelApplication.DisplayAlerts = False
excelWrkBook = excelApplication.Workbooks.Open(""c:\TestFolder\MAIN_Total.xlsx"")
excelWrkBook.SaveAs(Filename:=""c:\TestFolder\MAIN_Total.txt"", FileFormat:=Microsoft.Office.Interop.Excel.XlFileFormat.xlTextMSDOS)
excelWrkBook.Close()
excelApplication.DisplayAlerts = False
excelApplication.Quit()
Catch ex As Exception
Throw ex