dataGridView1.Rows[rowIndex].Cells[columnIndex].Value = value;

dataGridView1.Rows[rowIndex].Cells[columnIndex].Value = value;

this.dataGridView1.Rows.Add();
this.dataGridView1.Rows[0].Cells[1].Value = "1";
this.dataGridView1.Rows[0].Cells[2].Value = "Baqar";

// dgvBill is name of DataGridView

string StrQuery;
try
{
    using (SqlConnection conn = new SqlConnection(ConnectingString))
    {
        using (SqlCommand comm = new SqlCommand())
        {
            comm.Connection = conn;
            conn.Open();
            for (int i = 0; i < dgvBill.Rows.Count; i++)
            {
                StrQuery = @"INSERT INTO tblBillDetails (IdBill, productID, quantity, price,  total) VALUES ('" + IdBillVar+ "','" + dgvBill.Rows[i].Cells[0].Value + "', '" + dgvBill.Rows[i].Cells[4].Value + "', '" + dgvBill.Rows[i].Cells[3].Value + "', '" + dgvBill.Rows[i].Cells[2].Value + "');";
                comm.CommandText = StrQuery;
                comm.ExecuteNonQuery();
             }
         }
     }
 }
 catch (Exception err)
 {
     MessageBox.Show(err.Message  , "Error !");
 }

//dataGridView1.Rows[RowCount].Cells[0].Value = FEString3;//This was not adding Stringed Numbers like "1","2","3"....
DataGridViewCell NewCell = new DataGridViewTextBoxCell();//Create New Cell
NewCell.Value = FEString3;//Set Cell Value
DataGridViewRow NewRow = new DataGridViewRow();//Create New Row
NewRow.Cells.Add(NewCell);//Add Cell to Row
dataGridView1.Rows.Add(NewRow);//Add Row To Datagrid