Source

File: wp-admin/includes/update.php

function undismiss_core_update( $version, $locale ) {
	$dismissed = get_site_option( 'dismissed_update_core' );
	$key       = $version . '|' . $locale;

	if ( ! isset( $dismissed[ $key ] ) ) {
		return false;
	}

	unset( $dismissed[ $key ] );
	return update_site_option( 'dismissed_update_core', $dismissed );
}