Modify excel cell

although this is answered I'll add from my experience.

It is easier to open the ExcelPackage from FileInfo instead of Stream, then saving becomes simpler.

FileInfo file = new FileInfo(path);

        using (var package = new ExcelPackage(file))
        {
            ExcelWorkbook workBook = package.Workbook;
            ExcelWorksheet currentWorksheet = workBook.Worksheets.SingleOrDefault(w =>  w.Name == "sheet1");

            int totalRows = currentWorksheet.Dimension.End.Row;
            int totalCols = currentWorksheet.Dimension.End.Column;

            for (int i = 2; i <= totalRows; i++)
            {                   
                try
                {
                    currentWorksheet.Cells[i, 1].Value = "AAA";

                }
                catch (Exception ex)
                {
                    _logger.Error(String.Format("Error: failed editing  excel. See details: {0}", ex));
                    return;
                }
            }

            package.Save();

You can use the Interop dll's from Microsoft to edit office documents http://msdn.microsoft.com/en-us/library/15s06t57.aspx. Add the "Microsoft.Office.Interop.Excel.dll" to your solution. With this code i've changed 2 cell values.

static void Main(string[] args)
{
        Application excel = new Application();

        Workbook workbook = excel.Workbooks.Open(@"C:\Users\Martijn\Documents\Test.xlsx", ReadOnly: false, Editable:true);
        Worksheet worksheet = workbook.Worksheets.Item[1] as Worksheet;
        if (worksheet == null)
            return;

        Range row1 = worksheet.Rows.Cells[1, 1];
        Range row2 = worksheet.Rows.Cells[2, 1];

        row1.Value = "Test100";
        row2.Value = "Test200";


        excel.Application.ActiveWorkbook.Save();
        excel.Application.Quit();
        excel.Quit();
    }

I've started with Test1 and Test2 wich after the program running changed into the proper values.

The problem in both cases is that the modified workbook is not saved back to the stream:

MemoryStream ms = new MemoryStream();
using (FileStream fs = File.OpenRead(@"Path\Test.xlsx"))
using (ExcelPackage excelPackage = new ExcelPackage(fs))
{
    ExcelWorkbook excelWorkBook = excelPackage.Workbook;
    ExcelWorksheet excelWorksheet = excelWorkBook.Worksheets.First();
    excelWorksheet.Cells[1, 1].Value = "Test";
    excelWorksheet.Cells[3, 2].Value = "Test2";
    excelWorksheet.Cells[3, 3].Value = "Test3";

    excelPackage.SaveAs(ms); // This is the important part.
}

ms.Position = 0;
return new FileStreamResult(ms, "application/xlsx")
{
    FileDownloadName = "Tester.xlsx"
};