Just override etc/view.xml file of catalog module and do changes as per below,

app/design/frontend/{Packagename}/{themename}/etc/view.xml

<vars module="Magento_Catalog">
 <var name="gallery">
       <var name="navdir">vertical</var>
 </var>
</vars>

Just override etc/view.xml file of catalog module and do changes as per below,

app/design/frontend/{Packagename}/{themename}/etc/view.xml

<?xml version="1.0"?>
<view xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Config/etc/view.xsd">
    <vars module="Magento_Catalog">
         <!-- for product page thumbnail -->
         <var name="gallery">
            <var name="navdir">vertical</var>
         </var>
         <!-- for fullscreen thumbnail -->
         <var name="fullscreen">
            <var name="navdir">vertical</var>
         </var>
    </vars>
</view>