Rule source on GitHub

DowngradeCovariantReturnTypeRector

Make method return same type as parent

 class ParentType {}
 class ChildType extends ParentType {}

 class A
 {
     public function covariantReturnTypes(): ParentType
     {
     }
 }

 class B extends A
 {
-    public function covariantReturnTypes(): ChildType
+    /**
+     * @return ChildType
+     */
+    public function covariantReturnTypes(): ParentType
     {
     }
 }

Configure your rector.php:

<?php

use Rector\Config\RectorConfig;
use Rector\DowngradePhp74\Rector\ClassMethod\DowngradeCovariantReturnTypeRector;

return RectorConfig::configure()
    ->withRules([
        DowngradeCovariantReturnTypeRector::class,
    ]);