public static void InsertText(string docName, string text, uint rowIndex, 
  string columnName)
{
  // Open the document for editing.
  using (SpreadsheetDocument spreadSheet = 
    SpreadsheetDocument.Open(docName, true))
  {
    Workbook workBook = spreadSheet.WorkbookPart.Workbook;

    WorksheetPart worksheetPart = workBook.WorkbookPart.
      WorksheetParts.First();

    SheetData sheetData = worksheetPart.Worksheet.
      GetFirstChild<SheetData>();

    // If the worksheet does not contain a row with the specified
    // row index, insert one.
    Row row;

    if (sheetData.Elements<Row>().Where(
      r => r.RowIndex == rowIndex).Count() != 0)
      // At this point I am expecting a match for a row that exists
      // in sheet1 but I am not getting one

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Spreadsheet;
using System.Xml;
using System.IO;
using System.Diagnostics;

namespace OpenXMLWindowsApp
{
    public class OpenXMLWindowsApp
    {
        public void UpdateSheet()
        {
            UpdateCell("Chart.xlsx", "20", 2, "B");
            UpdateCell("Chart.xlsx", "80", 3, "B");
            UpdateCell("Chart.xlsx", "80", 2, "C");
            UpdateCell("Chart.xlsx", "20", 3, "C");

            ProcessStartInfo startInfo = new ProcessStartInfo("Chart.xlsx");
            startInfo.WindowStyle = ProcessWindowStyle.Normal;
            Process.Start(startInfo);
        }

        public static void UpdateCell(string docName, string text,
            uint rowIndex, string columnName)
        {
            // Open the document for editing.
            using (SpreadsheetDocument spreadSheet = 
                     SpreadsheetDocument.Open(docName, true))
            {
                WorksheetPart worksheetPart = 
                      GetWorksheetPartByName(spreadSheet, "Sheet1");

                if (worksheetPart != null)
                {
                    Cell cell = GetCell(worksheetPart.Worksheet, 
                                             columnName, rowIndex);

                    cell.CellValue = new CellValue(text);
                    cell.DataType = 
                        new EnumValue<CellValues>(CellValues.Number);

                    // Save the worksheet.
                    worksheetPart.Worksheet.Save();
                }
            }

        }

        private static WorksheetPart 
             GetWorksheetPartByName(SpreadsheetDocument document, 
             string sheetName)
        {
            IEnumerable<Sheet> sheets =
               document.WorkbookPart.Workbook.GetFirstChild<Sheets>().
               Elements<Sheet>().Where(s => s.Name == sheetName);

            if (sheets.Count() == 0)
            {
                // The specified worksheet does not exist.

                return null;
            }

            string relationshipId = sheets.First().Id.Value;
            WorksheetPart worksheetPart = (WorksheetPart)
                 document.WorkbookPart.GetPartById(relationshipId);
            return worksheetPart;

        }

        // Given a worksheet, a column name, and a row index, 
        // gets the cell at the specified column and 
        private static Cell GetCell(Worksheet worksheet, 
                  string columnName, uint rowIndex)
        {
            Row row = GetRow(worksheet, rowIndex);

            if (row == null)
                return null;

            return row.Elements<Cell>().Where(c => string.Compare
                   (c.CellReference.Value, columnName + 
                   rowIndex, true) == 0).First();
        }


        // Given a worksheet and a row index, return the row.
        private static Row GetRow(Worksheet worksheet, uint rowIndex)
        {
            return worksheet.GetFirstChild<SheetData>().
              Elements<Row>().Where(r => r.RowIndex == rowIndex).First();
        } 
    }
}