Hi ,
I want to show exisiting excel file.
I want when I press button on form, then my existing excel file open in excel application.

This will open the Excel spreadsheet with the defaul ".xlsx" handler. You just need to ensure they have Office installed:

private void button4_Click(object sender, EventArgs e)
    {
      System.Diagnostics.Process.Start(@"C:\Data\Spreadsheet.xlsx");
    }
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