public Excel.Application oXL        = null;
public Excel._Workbook oWB        = null;
public Excel.Sheets excelSheets        = null;    

try 
{
    oXL = new Excel.ApplicationClass();        // Run the Excel Application

    string workbookPath = txtfile.Text;        // Get the Filename

    // Opening the Excel Workbook
    oWB = oXL.Workbooks.Open(workbookPath,
        0, false, 5, "", "", false, Excel.XlPlatform.xlWindows, "",
        true, true, 0, true, false, false);

    excelSheets = oWB.Worksheets;

    // Changing the format of the Excel
    foreach (Excel.Worksheet worksheet in excelSheets)
    {
        //Change the format of the Sheet.....

    }            

    // Output Filename
    string newFileName    = "testing.xls";

    // Saving the formatted Excel
    oWB.SaveAs( newFileName,Excel.XlFileFormat.xlWorkbookNormal, 
            null,null,false,false,Excel.XlSaveAsAccessMode.xlShared,
            false,false,null,null,null);                                

    // Show Success Message
    MessageBox.Show("Your file is successfully changed!","SUCCESS",MessageBoxButtons.OK,MessageBoxIcon.Information);        

}
catch (Exception exc)
{                
    MessageBox.Show(exc.Message,"ERROR",MessageBoxButtons.OK,MessageBoxIcon.Error);
}
finally 
{
    // Need all following code to clean up and extingush all references!!!
    oWB.Close(false,null,null);    
    oXL.Workbooks.Close();
    oXL.Quit();                                

    System.Runtime.InteropServices.Marshal.ReleaseComObject (excelSheets);
    System.Runtime.InteropServices.Marshal.ReleaseComObject (oWB);
    System.Runtime.InteropServices.Marshal.ReleaseComObject (oXL);

    oWB        = null;                
    excelSheets    = null;
    oXL        = null;

    GC.Collect(); // force final cleanup! 
}