remove_action.php

<?php

//
// Original
//

class Plugin_Class {
	public function __construct() {
		add_action( 'my_action', array( $this, 'action_method' ), 10 );
	}

	public function action_method() {
		echo "<p>I'm in a class!<p>";
	}
}

$pluginClass = new Plugin_Class();

// output
do_action( 'my_action' );

//
// Modified action
//

remove_action( 'my_action', array( $pluginClass, 'action_method' ), 10); // unload class action

// add new action
add_action( 'my_action', function() {
	echo "<p>I'm the replacement!</p>";
} );

// output with modification
do_action( 'my_action' );