If product has tier prices we can create dropdown in cart .. otherwise there will be standart QTY box

first update default.phtml … in chcekout/cart/item/default.phtml .. with code below

<?php
    $ID = $_item->getProductId();
   
     $_product =  Mage::getModel('catalog/product')->load($ID);

     $prices = $_product->getTierPrice();
     if($prices!=NULL){ 
              echo '<select id="selectqty" onchange="getNewVal(this);" style="margin-top:13px;border:1px solid;height:30px;">';
              echo ' <option value="'.$this->getQty().'">'.$this->getQty().'</option>';
            foreach($prices as $tier){
                  // echo intval($tier[price_qty]).'<br/>';
                   echo '<option value="'.intval($tier[price_qty]).'_'.$_item->getId().'">'.intval($tier[price_qty]).'</option>';
             } 
      echo '</select>';
      ?>
       <input style="visibility:hidden;" type="text" id="<?php echo $_item->getId() ?>_QTY" name="cart[<?php echo $_item->getId() ?>][qty]" value="<?php echo $this->getQty() ?>" 
         size="4" title="<?php echo $this->__('Qty') ?>" class="input-text input-block-level qty" maxlength="12" />
          <?php
             }else{
            ?>
        <input type="text" id="<?php echo $_item->getId() ?>_QTY" name="cart[<?php echo $_item->getId() ?>][qty]" value="<?php echo $this->getQty() ?>" 
         size="4" title="<?php echo $this->__('Qty') ?>" class="input-text input-block-level qty" maxlength="12" /> 
             <?php 
                }
            ?>

and at the bottom add this

next.. update cart.phtml in checkout/cart.phtml

find button “Update Shopping Cart” and add id=”update_cart”

 <button type="submit" id="update_cart" name="update_cart_action" value="update_qty" title="<?php echo $this->__('Update Shopping Cart'); ?>" 
    class="btn btn-link update"><i class="fa fa-rotate-right"></i> <?php echo $this->__('Update Shopping Cart'); ?></button>