Open XML SDK 2.0 - how to update a cell in a spreadsheet?

Here is the working code. This is a prototype. For a larger number of changes, one might open the document only once. Also, there are some hard-coded things like sheet name and cell type that would have to be parameterized before this can be called production-ready. http://openxmldeveloper.org/forums/4005/ShowThread.aspx was very helpful.

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();
        } 
    }
}

I've been working with excel and found this helper library to be of great help (I've created my own helpers for word, would have saved at least 2 weeks if I was aware of this): https://www.nuget.org/packages/SimpleOOXML/

This is what is needed to update cell (writer.PasteText(...)):

MemoryStream stream = SpreadsheetReader.Create();
SpreadsheetDocument doc = SpreadsheetDocument.Open(stream, true);
WorksheetPart worksheetPart = SpreadsheetReader.GetWorksheetPartByName(doc, "Sheet1");
WorksheetWriter writer = new WorksheetWriter(doc, worksheetPart);

writer.PasteText("B2", "Hello World");

//Save to the memory stream
SpreadsheetWriter.Save(doc);

byte[] result = stream.ToArray();
FileStream file = new FileStream(@"D:\x1.xlsx", FileMode.Create);
file.Write(result, 0, result.Length);
file.Close();

The Code posted by @CDonner throws some exceptions, i have added some of the code that will take care of code, which throws an Exceptions, here it is

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 Application.Model{
public class TempCode
{
    public TempCode()
    {
        UpdateCell("E:/Visual Studio Code/Book1.xlsx", "120", 1, "A");
        UpdateCell("E:/Visual Studio Code/Book1.xlsx", "220", 2, "B");
        UpdateCell("E:/Visual Studio Code/Book1.xlsx", "320", 3, "C");
        UpdateCell("E:/Visual Studio Code/Book1.xlsx", "420", 4, "D");
        UpdateCell("E:/Visual Studio Code/Book1.xlsx", "520", 5, "E");

        ProcessStartInfo startInfo = new ProcessStartInfo("E:/Visual Studio Code/Book1.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, "Sheet2");
            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){
            return null;
        }
        string relationshipId = sheets.First().Id.Value;
        WorksheetPart worksheetPart = (WorksheetPart)document.WorkbookPart.GetPartById(relationshipId);
        return worksheetPart;
    }


    private static Cell GetCell(Worksheet worksheet, string columnName, uint rowIndex)
    {
        Row row;
        string cellReference = columnName + rowIndex;
        if (worksheet.Elements<Row>().Where(r => r.RowIndex == rowIndex).Count() != 0)
            row = worksheet.GetFirstChild<SheetData>().Elements<Row>().Where(r => r.RowIndex == rowIndex).FirstOrDefault();
        else{
            row = new Row() { RowIndex = rowIndex };
            worksheet.Append(row);
        }

        if (row == null)
            return null;

        if (row.Elements<Cell>().Where(c => c.CellReference.Value == cellReference).Count() > 0) {
            return row.Elements<Cell>().Where(c => c.CellReference.Value == cellReference).First();
        }
        else{
            Cell refCell = null;
            foreach (Cell cell in row.Elements<Cell>()){
                if (string.Compare(cell.CellReference.Value, cellReference, true) > 0){
                    refCell = cell;
                    break;
                }
            }
            Cell newCell = new Cell() {
                CellReference = cellReference, 
                StyleIndex = (UInt32Value)1U

            };
            row.InsertBefore(newCell, refCell);
            worksheet.Save();
            return newCell;
        }
    }
}

}