If you have to upgrade your Magento version from 2.1.7 to 2.3.4 then follow the below steps.

Note: Please take the backup of your existing database and take a full backup.

After taking backup follow the below upgrade steps on the Magento root directory using the terminal.

Step 1 : php bin/magento maintenance:enable

Step 2 : Take backup using existing composer.json of your site’s Magento root folder

Step 3 : composer remove magento/product-community-edition –no-update

Step 4 : composer require magento/product-community-edition=2.3.4 –no-update

Step 5 : composer require –dev allure-framework/allure-phpunit:~1.2.0 friendsofphp/php-cs-fixer:~2.13.0 lusitanian/oauth:~0.8.10 magento/magento-coding-standard:~1.0.0 magento/magento2-functional-testing-framework:~2.3.14 pdepend/pdepend:2.5.2 phpunit/phpunit:~6.5.0 sebastian/phpcpd:~3.0.0 squizlabs/php_codesniffer:3.3.1 –sort-packages –no-update

Step 6 : composer remove –dev sjparkinson/static-review fabpot/php-cs-fixer –no-update

Step 7 : Open composer.json and add the below line in “autoload”: “psr-4” section in last.

"autoload": {
    "psr-4": {
        ...
        ...
        "Zend\Mvc\Controller\": "setup/src/Zend/Mvc/Controller/"
    },
    ...
}

Step 8 : composer update

Step 9 : php bin/magento cache:clean

Step 10 : rm -rf var/cache/* var/log/* var/page_cache/* var/session/* var/view_preprocessed/* generation/* pub/static/*

Step 11 : php bin/magento setup:upgrade

Step 12 : php bin/magento maintenance:disable

Thats it.

Now clean the cache and check your Magento version. It should be upgraded to 2.3.4