You can try like this :
static void Main(string[] args)
{
DataTable dt = GetTable();
DataRow[] dr = dt.Select("ID=2 and F_Name='raj'");
if (dr !=null)
{
foreach (var item in dr)
{
item["F_name"] = "rajan";
}
}
}
static DataTable GetTable()
{
DataTable dt = new DataTable();
dt.Columns.Add("ID");
dt.Columns.Add("F_name");
dt.Columns.Add("L_name");
dt.Rows.Add("1", "mit", "jain");
dt.Rows.Add("2", "raj", "patel");
dt.Rows.Add("3", "anki", "patel");
dt.Rows.Add("4", "alpa", "dumadiya");
return dt;
}