Cannot upgrade from 2.2.6 to 2.3 via composer
First be sure you are running PHP 7.1 or 7.2
Update your composer
run those commandes:
composer require magento/product-community-edition=2.3.0 --no-update
Specify additional packages for composer Run this
composer require --dev phpunit/phpunit:~6.2.0 friendsofphp/php-cs-fixer:~2.10.1 lusitanian/oauth:~0.8.10 pdepend/pdepend:2.5.2 sebastian/phpcpd:~3.0.0 squizlabs/php_codesniffer:3.2.2 --no-update
Remove unused packages
composer remove --dev sjparkinson/static-review fabpot/php-cs-fixer --no-update
Update autoload
Open composer.json
and edit the "autoload": "psr-4"
section to include "Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/"
:
"Magento\\Framework\\": "lib/internal/Magento/Framework/",
"Magento\\Setup\\": "setup/src/Magento/Setup/",
"Magento\\": "app/code/Magento/",
"Zend\\Mvc\\Controller\\": "setup/src/Zend/Mvc/Controller/"
Apply updates
composer update
Clear caches and generated content
rm -rf <Magento install dir>/var/cache/*
rm -rf <Magento install dir>/var/page_cache/*
rm -rf <Magento install dir>/generated/code/*
Update the database schema and data
php bin/magento setup:upgrade
Disable maintenance mode
php bin/magento maintenance:disable
Clear the cache, reindex, if you get a server error tray to deploy static content.
and that's all,
You can check the full explanation here
https://devdocs.magento.com/guides/v2.3/comp-mgr/cli/cli-upgrade.html