QWidget* some_cell_widget = myTable->cellWidget(0,1);                            // retrieve widget from cell
QDoubleSpinBox* dbl_spin_box = static_cast<QDoubleSpinBox*>(some_cell_widget);   // cast widget to double spin box
double val = dbl_spin_box->value();                                              // get value from spinbox

//To set the value:
int valueIWant = 5;
ui->tableWidget->cellWidget(0,2)->setProperty("value",valueIWant);"
//And to read the value:
int actualValue = ui->tableWidget->cellWidget(0,2)->property("value").value<int>();