Ok, I think that you should edit your "step" file of the "checkout" view of the template that you are currently using and change the code :
echo $this->cart->displayButton(JText::_('HIKA_NEXT'),'next',$this->params,hikashop_completeLink('checkout&task=step&step='.($this->step+1)),'if(hikashopCheckChangeForm(\'order\',\'hikashop_checkout_form\')){ if(hikashopCheckMethods()){ document.getElementById(\'hikashop_validate\').value=1; document.forms[\'hikashop_checkout_form\'].submit();}} return false;','id="hikashop_checkout_next_button"');
by
if($this->step==YOUR_STEP){
echo $this->cart->displayButton('Send Order','next',$this->params,hikashop::completeLink('checkout&task=step&step='.$this->step+1),'document.forms[\'hikashop_checkout_form\'].submit(); return false; ','id="hikashop_checkout_next_button"');
}else{
echo $this->cart->displayButton(JText::_('NEXT'),'next',$this->params,hikashop::completeLink('checkout&task=step&step='.$this->step+1),'document.forms[\'hikashop_checkout_form\'].submit(); return false;','id="hikashop_checkout_next_button"');
}
Note that you'll have to replace the "YOUR_STEP" by the step (1 2 3 4..) where you want to replace "next" by "send order"
Hope this will help you a little.