<?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="MagentoCmsControllerIndexIndex"
               type="[Name Space][Your Module]ControllerCmsIndex" />
</config>

namespace [Name Space][Your Module]ControllerIndex;

class Index extends MagentoCmsControllerIndexIndex
{
    public function execute()
    {
        // todo
    }
}