How to read an Excel sheet with multiple sheets, and edit only a single sheet

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C#

C#4.0

I have a program which picks up values from an XML sheet and writes them into an excel sheet, using TestWriter.

FileStream ostrm;
                    StreamWriter strm;
                    TextWriter txtWrt = Console.Out;
                    ostrm = new FileStream(path + "\\Output.xls", FileMode.Truncate, FileAccess.Write);

Now the output file, Output.xls has multiple tabs, but I only have to truncate and write into the first sheet, leaving the other tabs untouched. I was using the below code, but it deleted all other tabs and edited the first tab.

public Excel.Application app = null;
        public Excel.Workbook workBook = null;
        public Excel.Worksheet workSheet = null;
        app = new Excel.Application();
                workBook = app.Workbooks.Add();
                workSheet = (Worksheet)workBook.Worksheets.get_Item(1);
                workBook.SaveAs(path+ "\\Output.xls");
                workBook.Close();
                app.Quit();

I need to find a way by which this can be avoided.

Thanks in anticipation!