code: public void goToStep(int step) {
          for (int i = 0; i < getWidgetCount(); i++) {
              Widget w = getWidget(i);
              if (w instanceof MaterialStep) {
                  ((MaterialStep) w).setActive(false);
              }
          }
  
          Widget w = getWidget(step - 1);
          if (w instanceof MaterialStep) {
              ((MaterialStep) w).setActive(true);
          }
          setCurrentStepIndex(step - 1);
      }
docs: Go to specific step manually by setting which step index you want to go.