code: public void previous(final boolean skipSlideStep) {
          synchronized (this) {
              // Try to display the previous slide step first
              // If no slide step is remaining, display the previous slide
              if (skipSlideStep || this.selectedSlideModel.previousStep() && this.slidePosition > 0) {
                  this.slidePosition = Math.max(this.slidePosition - 1, 0);
                  displaySlide(getPresentationService().getPresentation().getSlides().getSlide().get(this.slidePosition), !skipSlideStep);
              }
          }
      }
docs: Go to previous slide.