Disable update notification of theme and plugin

place this code in functions.php

For theme update notification disable
add_filter( 'pre_site_transient_update_core', create_function( '$a', "return null;" ) );

For single plugin update notification disable
function stop_plugin_update( $value ) { unset( $value->response['really-simple-captcha/really-simple-captcha.php'] ); return $value; }
add_filter( 'site_transient_update_plugins', 'stop_plugin_update');

For all plugin update notification disable
remove_action( 'load-update-core.php', 'wp_update_plugins' );
add_filter( 'pre_site_transient_update_plugins', create_function( '$a', "return null;" ) );