Step 1) Enable the extension to specify config.php file from app/etc folder
<?php
return array (
'modules' =>
array (
'Learning_RewriteSales' => 1,
),
);
Step 2) Create Custom.php Class in Block
<?php
namespace Learning\RewriteSales\Block\Adminhtml\Order\View;
class Custom extends \Magento\Backend\Block\Template
{
}
Step 3) Create Info.php file to extend core Info.php
<?php
namespace Learning\RewriteSales\Block\Adminhtml\Order\View;
class Info extends \Magento\Sales\Block\Adminhtml\Order\View\Info
{
}
Step 4) Create di.xml file to specify which class you where extended ( Dependency Injection )
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">
<preference for="Magento\Sales\Block\Adminhtml\Order\View\Info" type="Learning\RewriteSales\Block\Adminhtml\Order\View\Info"/>
</config>
Step 5) Create a module.XML file to specify the setup version
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
<module name="Learning_RewriteSales" setup_version="2.0.0">
<sequence>
<module name="Magento_Sales"/>
</sequence>
</module>
</config>
Step 6) Create sales_order_view.xml file from Learning/RewriteSales/view/layout folder and write the below code
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="admin-2columns-left" xsi:noNamespaceSchemaLocation="../../../../../../../lib/internal/Magento/Framework/View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="order_info">
<action method="setTemplate">
<argument name="template" translate="true" xsi:type="string">order/view/info.phtml</argument>
</action>
</referenceBlock>
<referenceBlock name="order_info">
<block class="Learning\RewriteSales\Block\Adminhtml\Order\View\Custom" name="sales_order_view_custom" template="order/view/custom.phtml" />
</referenceBlock>
</body>
</page>
Step 7) Create the Custom.phtml and Info.phtml file from Learning/RewriteSales/view/layout and insert below code.
Custom.phtml
<h1>Hi, I am here!</h1>
Info.phtml
First Copy the code from Magento\Sales\view\adminhtml\templates\order\view\info.phtml file and past here after that add one more line
<?php echo $block->getChildHtml('sales_order_view_custom');?>
Step 8) Finally clean the cache and to see your custom code Sales Order View Page.
reference: How to extend backend template files in Magento 2