Append data to existing document

//the path of the file
string filePath = "C:\\ExcelDemo.xlsx";

//or if you use asp.net, get the relative path
filePath = Server.MapPath("ExcelDemo.xlsx");

//create a fileinfo object of an excel file on the disk
FileInfo file = new FileInfo(filePath);

//create a new Excel package from the file
using (ExcelPackage excelPackage = new ExcelPackage(file))
{
    //create an instance of the the first sheet in the loaded file
    ExcelWorksheet worksheet = excelPackage.Workbook.Worksheets[1];

    //add some data
    worksheet.Cells[4, 1].Value = "Added data in Cell A4";
    worksheet.Cells[4, 2].Value = "Added data in Cell B4";

    //save the changes
    excelPackage.Save();
}