Read the manual.

Here is one way to do it , I think, without the complications of COM. The GUI interface is probably not what you want but you can extract the ideas that document presents.

Where?

At OP:
You might want to use CSV-files from excel. The CSV files can easily be opened and edited with C++ because they are plain text, so no need for COM

Oops! link added.

I found the way myself. This is the code that works:

private: System::Void button7_Click(System::Object^ sender, System::EventArgs^ e)
{
String^ XlsFile = String::Format("Z:\\Documenti\\Lavoro\\Fatture\\2009\\Fattura_02_AEM_marzo_09.xls");
String^ XlsSheet = String::Format("Milano, {0}/{1}/{2}",
dateTimePicker1->Value.Day.ToString(),
dateTimePicker1->Value.Month.ToString(),
dateTimePicker1->Value.Year.ToString());

Excel::Application^ oXLApp;
Excel::Workbook^ oXLWBook;
//Excel::Sheets^ oXLSheet;
//Excel::Worksheet^ oXLWSheet;

Excel::Range^ rng;

//starts Excel application
oXLApp = (gcnew Excel::Application());
oXLApp->Visible = true;
//opens the workbook of the file "XlsFile"
oXLWBook = oXLApp->Workbooks->Open(XlsFile, 0, false, 5, "", "",
true,Excel::XlPlatform::xlWindows, "\t", true, false, 0, false, true, true);
//updates the range of sheet1, cell D4
rng = oXLApp->Range::get("D4","D4");
rng->Value2 = XlsSheet;

}