$ curl -O https://raw.githubusercontent.com/wp-cli/builds/gh-pages/phar/wp-cli.phar
$ sudo chown root.root wp-cli.phar
$ sudo chmod +x wp-cli.phar
$ sudo mv wp-cli.phar /usr/local/bin/wp
$ wp --info
PHP binary:	/usr/bin/php7.0
PHP version:	7.0.8-0ubuntu0.16.04.1
php.ini used:	/etc/php/7.0/cli/php.ini
WP-CLI root dir:	phar://wp-cli.phar
WP-CLI packages dir:	
WP-CLI global config:	
WP-CLI project config:	
WP-CLI version:	0.24.0
$ sudo wp cli update --allow-root
Success: WP-CLI is at the latest version.