The filter is there so you don’t need to edit wp-login.php
, simply use add_filter.
Put something like
function change_password_reset_mail_body($message, $key, $user_login, $user_data) {
return "click here: " . network_site_url( "wp-login.php?action=rp&key=$key&login=" . rawurlencode( $user_login ), 'login' );
}
add_filter("retrieve_password_message", "change_password_reset_mail_body", 10, 4)
In your theme’s functions.php.