Well, I thought it was successful, but no it was NOT. Because now I can't access neither ACP or the main index.
First, you should make a copy of your existing config.php file; keep it in a safe place! Next, delete all the existing phpBB files, you should leave your files/, images/ and ext/ directories in place, otherwise you will lose your file attachments, uploaded images and get errors due to missing extension files. You can leave alternative styles in place too. With this complete, you can upload the new phpBB files (see New installation for details if necessary). Once complete, copy back your saved config.php, replacing the new one. Another method is to just replace the existing files with the files from the full package - though make sure you do not overwrite your config.php file.