Using pre_post_update to modify $post->post_content doesn't work

Hi,

I wrote the following hook to replace the occurrence of “test” with “testing” . But once I press Update button I don’t see the content getting updated.

What am I missing? Can anyone please help me?

add_action( ‘pre_post_update’, ‘addrelnofollow_link’ );

function addrelnofollow_link ( $postid ) {
$post = get_post ($postid);
if ($post) {
$post->post_content = str_replace(“test”, “testing”, $post->post_content);
}
}

Best regards

anees k A