I created a new Ubuntu Drupal 8.1.3 on 14.04,But I want to drupal upgrade 8.2.5.

drush up drupal

root@drupal-512mb-sfo2-01:/var/www/html/drupal/sites/default# drush up drupal
Command pm-update needs a higher bootstrap level to run - you will need to invoke drush from a more functional Drupal environmen
to run this command.
The drush command 'up drupal' could not be executed. 
Drush was not able to start (bootstrap) the Drupal database. 
Hint: This may occur when Drush is trying to:
* bootstrap a site that has not been installed or does not have a configured database. In this case you can select another site
with a working database setup by specifying the URI to use with the --uri parameter on the command line. See `drush topic
docs-aliases` for details.
* connect the database through a socket. The socket file may be wrong or the php-cli may have no access to it in a jailed shell
See http://drupal.org/node/1428638 for details.

Drush was attempting to connect to: 
Drupal version : 8.1.3 
Site URI : http://default 
Database driver : mysql 
Database hostname : 127.0.0.1 
Database port : 
Database username : drupal 
Database name : drupal 
PHP configuration : /etc/php5/cli/php.ini 
PHP OS : Linux 
Drush script : /usr/local/bin/drush 
Drush version : 8.1.2 
Drush temp directory : /tmp 
Drush configuration : 
Drush alias files : 
Drupal root : /var/www/html/drupal 
Drupal Settings File : sites/default/settings.php 
Site path : sites/default 
Sync config path : sites/default/files/config_zWDi3fOuGc5C_D6ebw8cglPgBBXQJRq5Aznm2bexOjgdUTFATTXRQY-BHxerNyJBiq3ucYBh4 
Q/sync