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

I want to update a cell in a spreadsheet that is used by a chart, using the Open XML SDK 2.0 (CTP). All the code samples I have found insert new cells. I am struggling with retrieving the right worksheet.

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

When I navigate the tree in Visual Studio, I am seeing three sheets, but none of them has any children. What am I missing?