Why Upgrade to the new wordpress?
Its recommended to upgrade the wordpress newly version when you see notification on top bar of admin panel, Because in new version have fixed many of bugs and issues fixes of layout, panel UI, editor and introduce new features and API functions.
Why this error occurred?
On every release of wordpress, package file increased size by size currently I have got 6MB of newly version 4.3 so on slow internet connection its take too much time to download from his upgrade code, but default max execution set is 80 seconds, so lets its cannot download within 80 seconds then thrown an error.
Fatal error: Maximum execution time of 80 seconds exceeded in WP-DIRECTORY\wp-includes\class-http.php on line 1597
Solution
Its need to increase time to allow upgrade, I have set it 10 minutes go to Find file wp-config.php and add below code, and remove it after updates of wordpress.
ini_set('max_execution_time', 10*60); //10*60 =600-seconds = 10 minutes
