//Create the Connection String
//You must specify the Provider as is here, I think
//Thre Data Source is the path to your file
//Extended Properties is something to do with excel, use as here

string ConnectionString=@"Provider=Microsoft.Jet.OLEDB.4.0;
					Data Source=C:\Test Projects\Excel example\Excel - reading an excel file\ReadThis.xls;
					Extended Properties=Excel 5.0";

//Create the connection

System.Data.OleDb.OleDbConnection ExcelConnection = 
new System.Data.OleDb.OleDbConnection
				 (ConnectionString);
			
//create a string for the query

string ExcelQuery;

//Sheet1 is the sheet name
//create the query:
//read column with heading A from the Excel file

ExcelQuery = "Select A from [Sheet1$]"; // from Sheet1";

//use "Select * ... " to select the entire sheet
//create the command

System.Data.OleDb.OleDbCommand ExcelCommand = new System.Data.OleDb.OleDbCommand(ExcelQuery,ExcelConnection);
			
//Open the connection

ExcelConnection.Open();

//Create a reader

System.Data.OleDb.OleDbDataReader ExcelReader;
ExcelReader = ExcelCommand.ExecuteReader();
			
//For each row after the first
//Message box the values in the first column i.e. column 0

while (ExcelReader.Read())
{
  MessageBox.Show((ExcelReader.GetValue(0)).ToString());
}
ExcelConnection.Close();

//Try update the file

ExcelQuery = "Update [Sheet1$] set B = 2 where B = 1"; // from Sheet1";

//Create the command to be executed

ExcelCommand = new System.Data.OleDb.OleDbCommand
(ExcelQuery,ExcelConnection);

//Open the connection to the file

ExcelConnection.Open();

//Execute the update

ExcelCommand.ExecuteNonQuery();

//Close the connection

ExcelConnection.Close();