Hi,
You can use the code given by Xavier and Mohamed with a little modification
<?php if($this->step > 0){ ?>
<input type="button" value="Previous" onclick="window.location.href = '<?php echo hikashop_completeLink('checkout&task=step&step='.($this->step-1), false, false, true); ?>'; return false;"/>
<?php } ?>
But you have to add a class in the input and change your CSS in order to have the look you want.
You can't use the same class than the next button. The arrow is not display in the same direction.
So it require some customization from your side.
Regards,