How to save Datatable back to database in C#

I  have Datatable in which data is fetched by query from two tables in database.
Now I  have updated a value of  one particular column in Data Table.
I  want to save it back to database,but unable to do the same Please help
Here dtbPlate is my datatable in which I  already have values from two tables

                   

                        //Here dtbPlate is my datatable in which I  already have values from two tables
                       DataRow[] rowsToUpdate = dtbPlate.Select("TestCode ='" + iTestCode.ToString() + "'");
                      foreach (DataRow row in rowsToUpdate)
                      {
                          row["RLU"] = Convert.ToInt64(row["RLU"]) - avgBlanking;
                      }
                    
                      string selectStatement = "SELECT RLU FROM TblPlateSchedule";
                      SqlConnection objSqlConctn = new SqlConnection();
                      objSqlConctn.ConnectionString = "Server=(local);Database=DBLUMIQUANT;Trusted_Connection=True;";  //Assign connection String

                      if (objSqlConctn.State == System.Data.ConnectionState.Open)
                      {
                          
                      }
                      else
                      {
                          objSqlConctn.Open();
                      }
                       SqlDataAdapter sqlDa = new SqlDataAdapter();
                      sqlDa.SelectCommand = new SqlCommand(selectStatement, objSqlConctn);
                      SqlCommandBuilder cb = new SqlCommandBuilder(sqlDa);
                      sqlDa.UpdateCommand = cb.GetUpdateCommand();
                      sqlDa.Update(dtbPlate);

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