code: public void prevStep() {
          if (currentStepIndex > 0) {
              Widget w = getWidget(currentStepIndex);
              if (w instanceof MaterialStep) {
                  MaterialStep step = (MaterialStep) w;
                  step.setActive(false);
  
                  // prev step
                  int prevStepIndex = getWidgetIndex(step) - 1;
                  if (prevStepIndex >= 0) {
                      for (int i = prevStepIndex; i >= 0; i--) {
                          w = getWidget(i);
                          if (!(w instanceof MaterialStep)) {
                              continue;
                          }
                          MaterialStep prevStep = (MaterialStep) w;
                          if (prevStep.isEnabled() && prevStep.isVisible()) {
                              prevStep.setActive(true);
                              setCurrentStepIndex(i);
                              PreviousEvent.fire(MaterialStepper.this);
                              break;
                          }
                      }
                  }
              }
          } else {
              GWT.log("You have reached the minimum step.");
          }
      }
docs: Go to previous step , used by linear stepper.